2. The center differential is limited slip (Torsen), so it can always distribute at least a little power between the front and rear axles.
I'm not sure how it works on the 5th gen, but on mine the center lock in 4 Hi is convenient because I can activate it while moving, like if I'm approaching a section of mud. That way I don't have to stop, shift to neutral, and go into 4 low.
The rear differential is open, but ATRAC will pulse the brakes if one wheel starts slipping to act like a limited slip differential. If works very well in my experience.
3. The top speed in 4 low is around 55mph, but you should use it when you need more power or control, such as carefully crawling over rocks and roots.
Let's say you come to a stop with the front wheels resting on an obstacle, like a curb. If you are in 4 high and you try to go over it from a stop, you will spin up the torque converter and once it makes enough torque you will launch over it and go way too fast.
In 4 low there is much more torque from the gearing, so it only takes a light touch of the throttle to go over stuff. That's easier on the torque converter, and it keeps you from going too fast and braking stuff.
On dirt or gravel roads without big rocks and roots, 4 high is usually appropriate.
4. I'm not sure what you mean. The Limited is full-time 4wd, so it's always on. You can lock the center differential, and you should do that if you need more traction to get going or not get stuck. For example, I might lock the center differential to go through water or deep sticky mud, go through a snow bank, crawl over roots and rocks, etc. You don't want to have the center locked on dry pavement because the drivetrain will bind up if you try to turn, making it difficult to turn and possibly causing excess wear on tires and stuff.
5. It's much better in my opinion. The 4runner has an extremely strong 4wd drivetrain that allows it to crawl over obstacles on trails. It is able to send almost all of the engine power to a single wheel if needed*. The Limited also has the advantage of a limited slip center differential, so it's excellent on the road (unlike Jeeps and lower trim level 5th gens where the transfer case is always locked if you are in 4wd). The Torsen center differential is similar to Audi's Quattro system and other high end cars.
*You would need front and or rear locking differentials to send all the power to one wheel, but ATRAC does a very good job as it is.
Subaru has a variety of all wheel drive systems, and they are not forthcoming about how they work or which vehicles have which system. As far as I know, they are generally based on viscous coupling or electronic clutches, which will slip unlike mechanical locking. And they can wear out with excessive use. I think they have some ability to use the brakes like ATRAC does, but when I try to learn more all I find is hype.
6. A stock height 4runner can do way more than you would expect. With a lift you can gain a bit of clearance, and aftermarket bumpers will help the approach and departure angles. In my 4th gen I usually scrape stuff in the center, not the bumper, and I think it's about an inch lower than the 5th gen.
