Doesn't sound like you've checked the U-joint angles, they could be way off of ideal and cause the trouble. Also might be out of phase as already mentioned.
All those answers and more about driveshafts and U-joints can be found here http://www.stovebolt.com/techtips/driveline/setup/index.html

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-