I have owned this Merc. for two years, and noticed 12 months ago, that when the car is fully loaded (four passengers) and their luggage, that there is vibration when I accelerate. If the car is coasting, no vibration, and it is worse when I accelerate suddenly. MY mechanic(s) is at a loss how to fix the problem, and I know little about cars, so!!! I know that the universal joint was replaced (twice) and that several rubber "couplers" on the driveshaft were also replaced. Any Ideas?

Thanks
NLF

Sounds like you have a sedan instead of 300Sl. Possibly the car (with this much load) is setting too low in the rear. That can cause the car to shake. Maybe the rear springs are "sagging" & the rear subframe mounts are worn.