Todelo Ron

I have a 2000 E320 4matic wagon with about 114k miles on it. In general it is in very good condition. While traveling for the holidays the car died on the highway. The engine would run fine but not move the car forward. There was what appeared to be gear lube on the ground and in the engine compartment arround the passenger side of the underside of the car. It seems to me to be the front differential. We had it towed to a Mercedes dealer that was near by but that we have never dealt with. Does anyone have any experience with this failure? I would assume that the front differential will need to be rebuilt and resealed. What should this cost? What parts probably failed? Any help would be appreciated. Thanks.