My 1991 300SL had the vibration issue also. It was very pronounced just before each up shift and then at higher speed cruising. I first rotated the tires, as it seemed to be better after the car was driven a bit. But that didn't help. Next I replaced the flex couplings, as they showed signs of cracking, but that didn't help. It turned out to be the drive shaft hanger bearing. It's not easy to see, as on the 300SL, it was necessary to drop the exhaust pipes out of the way for access. But it cured the problem.

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