what is the condition of you wheel bearings, your rear axle shafts, and your brake rotors?
do you think it could possibly be that your drive shaft is out of balance?

it is something spinning along with the drive train and is only affected by the overall speed of the car. those are my suggestions for things to look for, let me know what comes up.

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.