I have a 91 4runner v6 with a 4" lift. When accelerating its sounds fine but at high speeds when i let off and it makes some crazy horrid vibration noises. It has no issues accelerating so i dont think its the tranny, but could be the u-joints or rear end. Anyone have any ideas?

 

Could be many different things or combinations of things. Probably hundreds of threads about driveline/drivetrain vibrations. Search

 

This is covered in a few other posts but likely suspects are some combination of the below:

worn u-joints/unbalanced drive shaft
rear diff oil level ( check at filler windo/is there any/see any leaks?)
Might be time to replace the bearings in the diff or around your axles

I'm not a professional, but I've resolved to fixing my noisy rear end