I bought my 2500 new in '96 with a V10 and the same NV tranny you are using. I had the same problem with vibrations as you are noting in the higher gears. I took the truck back to the dealer three times to fix this "problem" and they never solved the vibration: told me that was what it was designed to sound like. Not what I wanted to here but 150k miles later I have not had a single issue with it.

Regarding the earlier thread on the motor mounts I put a pair of 1 5/8" roll cage tubes between the frame rails and flared out the tubes on each end to hold the torque (1153 ft. lbs). I don't have your engine weight though.