MERCEDES M112 ENGINE:

To deal with the inherit vibration problems of a 90 degree V6, a balancer shaft was installed in the engine block between the cylinder banks. This essentially eliminated first and second order vibration problems. (when it's installed properly).

Good luck, could be a boat load of other things, but if the dealer says the car is firing fine and not throwing codes to the DRBIII, then you need to look inside at the balancer shaft.