Glad to hear the ordeal is ending, and that you seem to have found a reliable shop to have the car attended to when it needs it. I am also glad to find out it was not a Mercedes-Benz defect in either design or manufacture. It actually sounds like you may have had something replaced that was ok to start with considering you only had 50,000 miles or so on the car.
One thing I did note in all of this is, when a real professional goes to work on a car and a problem results from that work, even possibly through no fault of his own, the pain is shared with the customer and the experience gets turned into another reason to return to that establishment. I am refering to the line from Steve which I quote "The fact that I picked out this vibration problem quickly was the only gratifying part of doing the job over for free. "
If you can find someone like that to work on your vehicle you will be miles ahead of the rest of the crowd looking for cheap labor. Congratulations Steve. If I lived in the vicinity I would use your shop!
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)