[Richard]

I am guessing that the pinion is turning up to much under a load, partially because of the tall blocks. Try shimming the pinion down instead of up (just a couple of degrees). An add-a-leaf and a shorter block may help as well. Springs and no block would most likley cure the problem. I had an 85 shortbed with custom 10" leafs front and rear. Did not have a slip-yoke elim kit. Put around 60,000 miles on it with the lift, no vibration problems. Really no driveline problems at all once I used Spicer u-joints. The slip-yoke kit is a good upgrade though.