I'm going through this same issue with my car. I've replaced Flex discs, center support and bearing, motor mounts. All MB factory parts. Next up is trans mount, rear diff mount, centering bushings. If that doesnt fix it...then I'm going to change the universal joint. Its not the tires because the
vibration only occurs on acceleration. It is usually
ANY acceleration even the
slightest push on the accelerator causes it.
I can be cruising along at 30-60 mph and its smooth as silk....step on the peddle slightly and a vibration that you can feel through the seat happens.
Back off slightly to cruise again and its gone. The harder you accelerate....the more you can feel the vibration. Its a frustrating problem to narrow down. I will have rebuilt the whole driveline by the time I figure it out
John