I think there is a known problem with the differential on this car.

On my 96 W210, I've had to replace the rear differential seal 3 times since I've oned the car.
All 3 times were covered by warranty but it was still troublesome.
From my understanding, the last time they fixed it, they said they installed a newly designed seal which solved the leak and it has not leaked since then.
That might be your problem as well.