I'm working on a 94 4Runner V6, 4WD, Auto. I've noticed that it vibrates when the 4WD is engaged at ~20 MPH or faster (I haven't gone faster than ~30 MPH with it engaged). It works fine at low speeds. This weekend I greased the shaft and front-most U-joint and everything seemed tight with it (no flopping around). Is a high speed vibration normal?

As for progress, we've so far replaced a lot of interior stuff, sealed the valve cover leaks, replaced all the cooling hoses (and many vacuum hoses), installed a thermostat and flushed the cooling system, changed all the other fluids (PS, brake, ATF, Tcase, and diffs). We changed the filter dryer and expansion valve and charged the AC and it now works (FL is hot this time of year). I have a replacement blower coming to resolve the awful noise the current one makes. A while back I replaced the gauge cluster and VSS to get the speedo working. The 10K resister mod is on the EGR plug to keep the CEL off too.

We're getting closer to having a daily driver. Thanks in advance for any insight on this 4WD vibration.

I've been thinking about getting my AC working too. Did you do all the work yourself, or did you bring it to a shop?

You shouldn't have a lot of vibrations in 4wd (assuming you are on a smooth surface).
Where do they seem to be coming from?

nicnmike, I took it to a shop for the AC work. I don't have the tools or gas for that work. It ran about $700 for that repair...about twice what I'd hoped. Based on your location, you know what the weather is like around here. Sucks driving in the rain with the windows down.

fourwd1, It feels like it's coming from the front diff area, but that should be spinning regardless of whether 4WD is engaged right? The tires on it are more worn in the rear, but not greatly so. Maybe 1/16 of an inch difference. I've been pondering whether that's the culprit.

It could also be lack of use too. I was told by the previous owner that this was driven daily, but having gone through a lot of it I'm sure it sat at some point. It may be it just needs to be run. I'll try some light offroad and see whether it gets better or worse with use.

Yeah, the diff and drive shaft would be turning but with no load (unless the PO installed manual hubs).
But being under load can make a big difference.
A little more wear on tires shouldn't.

The front drive shaft won't be turning in 2wd, because the ADD disconnects things so only the spider gears in the diff turn - the ring and pinion are stationary. The CV shafts turn on both sides in 2wd as well as 4wd.

I agree there should be no noticeable vibration with 4wd engaged at speed if you're going straight. Could be a front u-joint, balance of the front drive shaft, or a loose pinion bearing.

I did a couple miles of low speed 4WD, but no change so far. Funny story though, I went out to change the lower bumper valence (mine had some damage) and had a big puddle of oil under the truck. I said some choice words and moved onto the 2011 for an ATF change that was due. Then I got back to looking at the 94 and discovered it was just the oil filter. It's a tight spot in there so I didn't get it tight enough with my hands. I gave it half a turn with a wrench and cleaned the mess up, and no more leak. I'm a doofus.
The bumper valence change was quick and easy.

Hmm, mine has vibrated in 4WD no matter what speed. interesting. I've been chasing a shaking issue in 2WD on the highway.. I wonder if this is connected.

Maybe. I have a slight, intermittent shake on the highway in 2WD. I've attributed that to the cheapo tires it came to me with. Overall, for a 20+ yr old truck with unknown true mileage it rides and drives well. It's also seen a significant improvement in how it runs and shifts after some fluid changes and use. I think the use made the most difference there. I've been driving it daily about 70 mi/day for the past two weeks. It actually seems to have a power band now and the shifting has smoothed out nicely.

On the shake though, I'm still baffled.

My shake was like that until I got 31s and it became more severe. Our issue sounds so similar. I thought it was my tires too. I wonder whats in common >.< I'm killing myself over this issue because I've replaced:

* rear axles
* steering stablizer
* pitman arm
* shocks
* all gear fluids except front diff (don't know how)
* cv axles
* hub bearings (both front and rear)
* brakes
* tires.

I tried tires/rims from another 4R and it shook then too.

I swapped around my wheels in a couple different ways yesterday to try to prove out my tire hypothesis, but it didn't pan out. There was no impact on it. It's tough to troubleshoot my 2WD issue because it's very slight and intermittent. Looking at your list of changes though, I have a thought. What do you think about changing the U-joints? I have had these go in a way that produced a pretty obvious shake at certain drive train RPMs (a specific speed range) and that may be what I'm experiencing here, but at a less noticeable level.

My next two weekends are full, but after that I think I'll do the U-joints on the rear shaft and see if that helps. Probably regardless though, the front shaft U-joints will be done too. My vibration up front in 4WD is very noticeable at ~30mph.

My theory is currently front U-joint DS, but the front shouldn't spin at all in 2WD. I'm checking mine this weekend regardless. I've changed my rear u-joints twice and had my driveshaft built. I have a really nice driveshaft out of a prerunner that was balanced on my flanges with new Moog joints. I agree. It does feel like U-joint shake, but it shook without the rear driveshaft in at all, in 4WD, on the interstate. I'll post my findings when I find 'em