Sounds a lot like the CV joint on one of the rear axles is bad. Get the car up on a lift and examine the rubber boots on the axles. There are four of them, two on each axle. When the rubber fails due to age (not likely in your case) or due to being struck by debris on the road (more likely) the grease comes out and water makes its way in. Eventually the joint begins to fail and it makes some horrible noises as it goes. Crunching and cracking noises are not uncommon.

If this is the problem I would not use the same mechanic. He apparently knows nothing about the driveline of an MB.

On newer MB's changing the half shafts with the CV joints is pretty straight forward. You might consider doing it yourself. Jim

__________________
Own:
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

Owned:
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)