I have a 2002ML320 and mine has done this off and on since I have had the car. Every time I have it serviced I mention this and they check/fix it. Of course it starts happening again, this last time (2 days ago) this is what is says on my repair sheet:

Cause: electrical
282330 Low Range Switchover Motor, R & R/REPL.
492 WM 93
1 163-540-08-88 electric mot

I am not really sure what that means, but once again the problem was fixed. I will just wait and see how long this lasts...