My SDL had a similar vibration before I had the driveshaft re worked with a new u-joint and balance.

It was subtle at first so I replaced flex disks, center bearing & carrier, engine - trans - diff mounts...no joy.

I then read about u-joint notching causing vibration especially under load at low speed. So I changed the drive shaft angle pitch by putting a simple washer between the center bearing carrier and the mount area on the car body thus changing the drive shaft alignment.. low and behold much less vibration. So I added another and it cleared up the low speed and under torque vibration for another 10,000 miles.

Eventually the vibration came back (notched u-joint) so had to have the drive shaft reworked.

__________________

1986 300SDL 440,xxx