have a 93 4Runner that when I bought had serious rear end howling issues. No Problem though since I was going to rebuild it anyways with new gears.

When we got the old rear apart, the previous owner had gone in and overtightened the crush collar on the pinion bearing, which was the cause of the high pitched howling noise.

Now to the present day, once the 5.29's were installed there was very minimal noise, at least nothing that concerned me too much since I expected a lower gear to omit some driveline whine. However, it seems that over time it has gotten worst. Between 65-70 the whine gets louder and a vibration starts to develop, and between 45-55 when you let off the gas a vibration along with a heavy chatter can be heard from the rear.

The truck has a 4" suspension lift so I wouldn't think that the driveshaft is at to steep of an angle. I know it could be the ring and pinion, but I am trying to explore all options before tearing it down again.

Feel free to throw your input and suggestions my way!

 

My 94 did that, it needed new u-joints, solved the same howling issue that you described.

 

You could measure the driveshaft angles and see if that is the problem:

http://www.4crawler.com/4x4/CheapTri...l#Measurements

And check the u-joints and slip yoke for play as well as the t-case output flange.