Does the vibration seem to match the speed of a rotating tire, or is it a much higher frequency?

Have you checked the frontend for tightness?

I would imagine that more often than not, tire balance is the cause.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles