Hold the transmission in 3rd gear, run the car until the engine RPM is the same as when you are doing 55 in "D".
Do you get any vibration?
If not, it is unlikely to be the engine, mounts or shocks.
When doing 55, slip the transmission into "N" and let the engine idle. Does the vibration stop? No? Then it is not the engine.

Check the front suspension, wheel balance (any weights fallen off?), flex discs and drive shaft.

__________________
Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.