MILO, start with a close visual inspection of the front and rear flex discs. Check for hairline cracks, shredding or elongation of the rubber at the sleeves. Just finished replacing the rear flex disc on our 1992 300CE which had a similar vibration. You'll likely need thin 41mm and 46mm wrenches to loosen the driveshaft coupling bolt if you replace the flex couplings yourself, but confirm the sizes beforehand.

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Fred Hoelzle