Driveline vibration help

So you ran an unloaded rear end on jackstands in effort to find an imbalance.? You saw the slip jopping up n down at approx 60. Was that under power with a light brake application? The brake apply would lightly simulate load ie. traveling on level ground at cruising speed. What you experienced may just be the driveline loading and unloading.
What did you mean by shock pads? The U-bolt caps? Or did they cut off the spring perches and relocate? Your pinion angle could be off if that had occurred.
Please be careful running your vehicle at speed on jackstands, **** can get sideways in the blink of an eye. Property damage would be the least of your concern then, loss of life becomes a real risk. A stone in a tire tread becomes a bullet, just sayin .... B safe