A small point
Not ultimate - but I do have the boss 2.5 series - so I'm not sure if this applies directly, but I'll add my 2 cents anyhow.
SAS articulates much differently than IFS.
The body of the truck follows more tightly the pitch of the front (...axle) than the back. (for example try this yourself: Take your swaybars off, and park one of your front tires on a curb and record your body roll. Do then do the same with the rear in the same spot. You should notice less body roll with the rear parked on the same curb. Some of this is a consequence of differing spring rates, some of this is a consequence of the differing load transfer characteristics of IFS vs. SAS).
Analyzing the load transfer dynamics of a vehicle can give you some insight into the consequences that having IFS in the front and SAS in the rear might have on the lean of the body compared to the articulation of a suspension. There is a virtual fulcrum of lateral load transfer (which is an imaginary point you can define for components in the suspension - but has typically a variable / hazy field of points when all factors contributing to dynamic load transfer for body / chassis are combined, unless every part perfectly resists deflection (in this case we might call it a roll axis)), which although I haven't actually done or seen the math for our trucks - I believe to intersect the cG nearer the front of our vehicles. More to the point - the roll center of the suspension is much nearer the ground on our IFS front, whereas it is directly above the pumpkin on the solid axle rear. A higher roll center in the rear means a more anterior intersection of the vehicle center with the roll axis. The net effect of all contributors is a higher roll stiffness in the front - and the body follows the articulation of the front suspension more than that of the rear.
Short version for our trucks: if you have some side to side lean in the front - it will likely translate to increased side to side suspension articulation in the rear (which will appear as a wider side to side gap difference between the tire and fender on the passengers side's rear than the front. (ie: 1" in the front may look like 2" in the back)
((((This is not always true however, as there are many variables which contribute to the output. It is fairly safe to say that a small sample size with many different combinations of variables could exhibit different outcomes that minimize the generalizability of the effect. It is also fair to say that this is a well documented, well described and certainly mathematically supported consequence that many forum members suffer from. Manufacturers are aware of this (as is Toyota most certainly) which is why some suspension kits have slightly different (taller / stiffer) springs for for the drivers side. The shocks themselves are identical, and don't need to be any different (though I will get into this in a bit below.... again unsurprisingly - you can invent a scenario why this might not be true and contribute to a functional consequence, but then we are just being pedantic... it is not a completely different ballgame). Adding: swaybars, variable spring rates, variable shock damping, X-Reas, and weight redistribution techniques allow us to adjust for the disparity and correct the result.))))
***** In answer to your question, crank up the front coilover until the gap between the tire on each side to the fender is the same. You will diminish the lean in the rear. This will improve further if you have a strong and functional swaybar, or rake the front of your vehicle. @%1;'s understanding here is supported as he said that all of the vehicles in his experience (save his own) are raked.
Raking the front will tend to diminish the rear side to side difference (because this moves the intersect of the roll axis forward with the vehicle center (where the vehicle center is the functional center of the most ant/post/lat mounting interfaces with the chassis), while leveling will tend to increase it (again because this moves the intersect of vehicle center and roll axis closer to the rear, ie: closer to the lateral disparity).
***** The few pounds of lateral disparity in weight distribution + driver weight, is actually a valid (and potentially significant) consideration here. If it is true when put onto a scale that there are 160 pounds more on the driver's side tires than than on the passenger side (as measured by scales under each wheel), then everything else being equal, the springs on that side are being compressed 1/2" in total (ie: sum of compression in the front and back) more than on the other side (if the standard 4runner spring rate was 320pounds/inch). This will increase with driver weight (though to be fair, 200 pounds does not go onto the driver's side tires in this case - there is of course redistribution - which can be roughly calculated if you know the starting points).
Adding a spacer is a fine idea, however a perfect solution would be to redistribute weight. There are of course other considerations - ie: the cumulative effect of manufacturing tolerances, as well as the fact pointed out earlier by @%1; that the 4runner was designed as a right hand drive vehicle, which both will have some effect on the lean (ie: this isn't only always because of a 160lb lateral disparity).
Toyota engineering is very very crafty, and more than any manufacturer that I've experienced, employs the most elegant and simple solutions to problems like this. (I might put a little thread together at some point - a... Toyota engineering appreciation thread if you will...). As a right hand drive car (and as soon as I have a chance I'll examine this) I expect that Toyota built subtle side to side differences into several aspects of the chassis, and with the change to left hand drive, these variances accentuate rather than correct the effect of driver weight. This was likely not corrected for us obtuse minded north americans in the chassis, but rather in the more cost effective measures of taller stiffer springs on the driver's side.
The absolute best solution is to move the cG closer to mid-line. This will have the side effect of reducing actual weight transfer due to body roll.
If you were to approach this problem from that angle - and you weigh 200 pounds (supposing the lateral disparity is something like 160 pounds) - then 200 pounds needs to be added to the passenger seat, and 160 pounds distributed over the upper mounts of the passenger side front and rear suspension to bring the VFLLT to midline. Of course this isn't very practical for a number or reasons, which I'll let you imagine. A more elegant solution would be to remove or transfer existing weight from one side to the other, or at least closer to center.
I might add...
Putting a spacer above the shock again is not the completely different ballgame vs. adjusting a coil-over to ride higher or lower (unless you are hitting the bumpstops). It has zero functional impact on the shock.
The working part of a shock is a piston sitting in an oil bath. Little holes in the piston allow it to move through the oil at a particular rate. Increase the oil viscosity - piston moves slower. Decrease the size of the holes - piston moves slower. Moving the shock through its travel by any amount has absolutely no impact in shock performance (unless of course you have progressive gas absorbers with variable rebound - increasing the compression will take away some of your fast rebound). Having a floating piston with a compressed gas rebound - again - no difference (but you might need to decrease the pressure above the floating piston to get the same damping performance). I should say that on lower end shocks, putting a spacer above the spring/shock hat might increase the deflection of the shock as it moves through its travel, and might put more strain on the superior mount - however; this is already accommodated for in most designs with the use of either a heim join at the upper mount (and then likely the lower mount) in the expensive designs, and a rubber bushing or two on the inexpensive designs.
Putting a spacer above a shock... is the same as adjusting a coil-over (caveats to the above) - so long as the total travel of the swing-arm at the lower mounting point of the shock does not exceed the upper or lower limit of the shock's travel. Adding a spacer diminishes some of shock travel at full compression, but increases it at full extension (adjusting the coilover does the opposite). So long as you do not exceed the travel of the shock through the travel of the swing-arm, they are functionally identical suspension adjustments (again - caveats to the above). There is no such thing fortunately as the shocks [being] different lengths. When you put more weight on one side vs. the other - the travel sags into the heavier side more - so in that case, at rest the shocks are at different lengths in their travel comparatively. If you have driver's side lean, you should absolutely do something to correct this. Adding a spacer effectively evens them out - so that the frame mount to swingarm measurement is the same distance on both sides. The spring is just loaded a bit more on the heavy side.
TLDR - spacers are a valid solution - for an observed problem that has many variables. Spacers do not contribute anything unreasonable to a functional compromise in suspension performance (but read the above before you flame).
Best advice is to start by laterally leveling the front, and see where you are before ordering anything for the rear.
