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?