91 Drive Train Vibration

I have a 91 GMC Safari RWD. Vibration is noticeable while accelerator is pressed only a little, between about 40-70MPH; when I hit 70MPH the vibration is constant -- I am scared to go any faster than that. I have put it in Neutral while traveling about 60MPH and reved it -- the vibration is present then also. Had the rear end up on jack stands with serious vibration at low RPMs in Drive and Reverse. I have had the drive shaft rebalanced after replacing both U-joints. Drained and replaced gear oil in differential. Previous to the problem, I have had a rebuilt transmission installed, a brake job, and new front wheel bearings. The motor and transmission mounts look okay. Any Ideas? Harmonic Balancer? Rear Bearings? Someone help me!!!

I owned an 87 Safari which developed a rumble in the drivetrain that proceded to get worse over time. The symptoms were very similar to what you describe. I ended up swapping out the entire rear end. That really isn't as challenging as it may sound.
You may want to start your trouble-shooting by examining the U-joints. Grab the propellor shaft and give it a few good hard jerks back and forth. If there is any obvious sloppiness or displacement in the U-joints this should show it. However, you may have to remove the shaft to check the needle bearings and if there is any grease left inside the cups. Take care when removing the cups as to not lose any of the bearings should they be loose. If they appear damaged or dry, I would recommend replacement. these need to be pressed in, so unless you have a hydraulic press, take your propellor shaft to a machine shop and they can do this for you.
If you do not think that this is the problem, you may want to check around some salvage yards to see if you can find a used rear end. (A new third member - ring and pinion assy. can cost over $800 installed!) It only took about 3 hours to accomplish and that included removal of my old one and a pull-it-yourself job at the junk yard and then replacing it.
Good luck.