Measure the driveshaft/pinion angle and then install some shims to correct it.

Since you are getting vibes when letting of the gas, my guess is the pinion angle is a little too low. Pinion naturally tips up under load, but when you back off the gas and coast or decelerate, the pinion drops back down and vibrates. Could also be looseness in the pinion or t-case flanges, see if you can feel any play in either flanges.