It would be useful to differentiate between a wheel vibration and a DL vibration, as their frequency will differ by a factor of 3. An axle problem might seem like DS vibration also, as the three-lobed spider in concert with a 3.07 final drive ratio will offer the same frequency as an OOB driveshaft. To possibly eliminate the axles as a problem, add weight to the trunk so that the axles are running perfectly straight and see what happens.

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'83 300D, 126K miles.