Scuba,
Don't know what year or milage your car is but I have seen other people on this forum check out their differential fliud and have found it to be in bad shape and changed it.
I would say ask a local Benz dealer or mechanic what they would recommend. I only have 20K on mine but plan on getting the tranny and dif serviced this coming summer by a Mercedes dealer if it doesn't cost me too much. Probably cheaper than a new dif or tranny. Never know how well a previous owner treated the car !