Check flex disc first. Check rear diff mounts and if problem still not resolved, probable excessive play in the LB3 clutches from reverse.
Try going from D to R quickly and then do it with a pause in neutral. If more severe doing it quickly, probably LB3 (reverse Clutches).
How many miles on the vehicle? How long does it take for reverse to apply.
Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture