I agree that the pinion angle is the problem. I had a similar problem after reinstalling a transmission a few years ago. The crossmember for the tranny had 2 sets of holes and I mistakenly used the top holes during install. When I let out the clutch the entire car shook. Looked like the windshield was going to come off. I immediately knew what I had done, went back and raised the crossmember to the proper holes. No problem.
Take a look at the alignment of the transmission, driveshaft, and u-joints. You'll probably see a significant angle at the u-joints. You might need a spacer between the trans. and the crossmember. You need to minimize the alignment angles at the u-joints. Probably an easy fix.
