Have you had your M-B dealer check it out? They have dynamic balancing equipment to spin the wheels while still on your car. This was the only way my front wheel high speed vibration problems were cured. It seems Mercedes-Benz found out about this problem and made the dealers get the dynamic balance equipment. I'm assuming the fault is not with the wheels and tires otherwise the ususal static balance would work. Must be a design problem??
Good luck and let us know if they have the equipment and it fixes the problem.
"Lead, follow, or better yet, get out of the way!"E500 owners motto