J-bird...... here's part of a post from the M-Class news group that might help.

http://www.escribe.com/automotive/european/mbmc/index.html

I would guess that the tow truck driver was right in suspecting a drive
axle issue. A number of owners on the list have experienced center
support bearing failure - though I haven't heard of any failures as abrupt
as yours. Most commonly, people in areas with harsh environments (salted
roads etc) report a very noticeable vibration. If you're not familiar
with a center support bearing, it is located halfway along the rear
driveshaft between the transfer case output and rear differential input -
it supports a long driveshaft and enables a change in angle. My guess
could be wrong, but I wouldn't be surprised to hear that this bearing
failed catastrophically on your vehicle. If this is the case, the repair
should not be too terribly expensive - perhaps somewhere in the $200 to
$400 range.