Grease Monkey does a 16 point inspection and I’ve found that should include the differential but I can’t find the details on the GM inspection points. The paperwork is in the RV. I’ll have to see what it says.
Edit:
I found this online for Grease Monkey.
Its 16-point inspection consists of the following checks and maintenance procedures: wash exterior windows, vacuum interior, check/fill battery fluid, check air filter, check windshield wiper blades, check breather element and PCV valve, check/fill windshield washer fluid, check all exterior lights, check/fill transmission or trans-axle fluid, check radiator level and test coolant, check/fill power steering fluid, install new oil filter, change oil (up to 5 quarts), lubricate chassis, check/fill differential fluid, and check and set tire pressure. Grease Monkey technicians also visually inspect hoses and belts.