Have y'all checked driveline angles and u-joint phasing as recommended by the Spicer driveshaft manual found in this tech tip.
http://www.stovebolt.com/techtips/driveline/setup/index.html

Just a guess but perhaps not enough angle on the first shaft, setting the joints in that first shaft out of phase might make a difference... just a guess, read the manual, check the angles, and see if anything looks likely.

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-