I had the same problem with my 560sec - shimmy and rough between 55 and 70 more or less. I took the car to a local shop for a wheel balance and it was worse! Called my Indie and he said only one shop in town knows how to balance MB wheels and sent me there. Afterwards it was like I had a new front end - smooth as silk and no more problems.
Good luck.

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles