Mechanic Q: Please noterespond to issue below?

had front & rear axle fluids changed @ 30,000, and now @ 38,000 am told by another facility that the rear differential fluid is "dirty/contaminated" needing service. Is this a different issue?? Also your service minder states the "crankcase" fluid needs to be changed at the current mileage.. Please help me with this confusing terminology... Thank you...