Make sure the wheel are true...not bent at the rim or even warped. We have a 560 SEL and a 300 SD and the suspension on both of those cars has never been an issue except whne I hit a curb and bent a rim and I had the same type of vibration you described.

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1990 560 SEL- "Goldielocks"
1984 300SD TurboDiesel- "Ole' Blue"