Wow, that is a great price if they're actually doing changes and not "inspections". A front and rear diff change alone at my dealer comes in at $260.My dealer just told me my next service, 30,000 mile would be $450. This includes front & rear differential fluid change, brake fluid change and oil & filter change.
If you have ever seen the metal that comes out of the diffs during the first change, yes, they need to be changed at 30,000 mi. After that I think they can be pushed much longer.