The old thread about this has gotten a little stale so I thought you might be interested in what we finally found. It's been a long journey to arrive at a simple solution. We've changed the drive shaft, moved the pinion angle around, checked the axles for run out, balanced everything that could be balanced, changed the angle on the trans, put on new tires (the old ties were 12 years old) We had just about given up and decided to pull the trans when Mike said let's try one more thing, he got a new 1310 yoke for the differential and presto, no more vibration. I have talked to a major 9 inch differential builder who has built over 100,000 differentials and he said he never had a differential vibrate. Guess there's a first time for everything. Just glad the problem is solved. Thanks Mike.