Do a search for RSN ( Rumble Strip Noise ) on some sprinter sites. The torque converter clutch may be dragging / being too aggressive causing a slip - stick - torsional vibration ( like being in too high of a gear in a standard trans )

The TCC on the 722.6 is modulated from 0 to about 98% lockup. When you are harder on the throttle the TCC isn't engaged as much so you don't feel the shudder.

A fix is to replace the TC with a spring release clutch. Changing fluid might not fix this. About the only additive I'd try is a friction modifier ( like posi traction additive ) called " Shudder Fixx " , it is made by " Lubeguard " Instant Shudder Fixx it is about $ 7 a tube. I've used this on many non MB cars and it works, as long as the problem is a TC shudder not a failing trans.