I would suspect a driveshaft vibration possibly made worse by a bad trans mount.

Check your flex discs for cracking/loose bolts. Check the center support to see that the flexible rubber mount is intact. Look at the u-joint for rust or looseness.

Also look at your drive axles for verticle play. This is a judgement call so your should see a professional probably.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician