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