1994 Chevrolet Suburban rear end

Replaced rear end with a used one
mechanic said mine was out of grease now in 4 wheel drive it goes sideways cannot be driven in 4 wheel drive carried back to garage he said it had the same gear ratio but that being out grease caused the differential to wear now that would need to be replaced is that possible I'm disabled on social security so my income is limited I wonder about this mechanic