Years ago I inquired with my MBZ dealer about replacing the ring and pinion in my diff (wanted a shorter gear), and they said they didn't have the tools, parts, or tech data to rebuild a diff. If a customer (warranty or customer paid) had an internal diff problem, they just replaced it with a new one. Don't know what the situation is today. Diffs are very reliable and rarely go bad inside.

The pinion seal can be replaced with the diff in the car. I replaced the pinion seal in my '63 Corvette back in the sixties - on the car and me on my back - not fun! I got much more enjoyment out of rebuilding the whole diff on the bench in the seventies.

If the "leak" isn't just weeping from the vent, the most likely suspect is the pinion seal.

Duke