Change the oil in the differential?

To me the main reason to change it is not because the fluid wears out or breaks down it is to inspect it to see what the wear looks like inside and to remove any metal particles that would increase the wear on the gears. There are folks that have never changed the fluid in the rear end or transmission for that matter with no problems and there are good mechanics that will disassemble the differential and check the gear pattern and see how it is wearing, solvent clean the inside of the box and reassemble. You should at least check the level and once the plug is removed it is not hard to pump out the old fluid and refill. Look for metal in the old oil.