U Joint Cap Size

If you put the car up on frame stands and let the rear sag on the leafs, you might be able to pick up the vibration when its in gear and someone is monitoring the speedo. Try it without the tires off to isolate the problem. Dont know if full rebound is going to make your pinion angle part of the problem though. Of course jackstands under the rear axle and no tires will isolate the tires too. Many times its just a thrown weight off a rim or a bulged/broken belt tire.