I cannot speak to the ML (since I don't know the details of the AWD differential system requirements), but I know many, many people who are running Mobil1 75W90 gear oil in their RWD W210s without any problems, myself included. Anecdotal evidence isn't science, but I've logged about 65,000 miles since changing it.
I didn't have any problems before and have had none since, and I have not heard of anyone using it that has experienced any issues, one of whom has been using it for nearly a decade (through at least one more fluid change) in his W210 diesel.
This product data sheet from Mobil1 indicates that it meets requirements under the MB Spec 211 sheet (from the bevo extranet MB site) for the ASTM D 445 standard:
Mobil 1 Synthetic Gear Lubricant LS 75W-90
Whether it would still meet the spec after the 20-hour test noted in the 211 spec sheet (CEC L-45-T-93,) is not clear.
I've attached the referenced 211 spec sheet for your entertainment.
Good luck.