I've been having a noticeable, in your seat vibration in my 1993 4Runner. You can really feel it at about 2k rpm in 3rd gear and cruising on the highway at 60mph. It's really more of a steady bouncing than a high-frequency vibration. It's also accompanied by a pulsation/wavering in the exhaust/engine sound. As well, I had her up on jackstands and gave her a run up to 3rd or 4th gear and every time I hit the gas there was a vibration from the rear half of the truck.

I recently tightened up my rear diff pinion flange, so that can be eliminated as a source of slop contributing to vibration. Could my driveshaft somehow be imbalanced? The output flange of the tcase for the rear shaft feels solid, so that probably isn't it.

I'm planning on putting her on stands again and removing the tires to eliminate those, and then the rear shaft to see if I can narrow it down. Just waiting on better weather.

 

Might not hurt to take the driveshaft off and clean out inside where the slip happens. Good and clean and then grease it again.

 

My thought was it might be a tire. I like your idea of running it on jack stands w/0 them. All u joints good? shocks good?

 

Turns out it was the tires. Removed them both and the vibration disappeared.

Thanks for your suggestions!