I have a 99 ML320 also, and had a simalr problem a couple of times. In my case it was after I had washed the truck and then parked it in the garage overnight with the parking brake applied.
When I attempted to drive it the next day there was a clunk and the car didn't want to move.
I soon discovered that the parking brake had locked up overnight due to the mositure from the previoius nights washing.
Now when I was my ML, I either take it for a short drive to remove the hidden water or I don't apply the emergency brake.

Next time you won't be driving the ML for a few days, try parking it on a flat surface and don't apply the emergency brake, you might get lucky and have the same problem I did.

Todd
99 ML320