Change the oil in the differential?

I do in all my vehicles.

I'm sure there are a lot of vehicles on the road that have never had the oil changed in the differential. But, that doesn't mean you should never change it.
It's good Preventative Maintenance.
And, it's not difficult or expensive so, I doubt your mechanic is doing it to get more money from you.
Did you check the owners manual? That should have the schedule for changing fluids.

Yes, change it as when it gets foamy it does not do the job it should!

15 years old time to change it.
Replace with synthetic and will be the last time you will need it done.
Unless you keep it for another 15 + years.

With a vehicle that age I would also change all belts/hoses/fluids period. You eliminate or reduce anyway many future problems. Tire check age/condition.

I think my manual calls for the diff. fluid and transfer case fluid to be changed every 50,000. Whatever it is that's what I have done. I'm sure there are lots of people out there who never do it. There's probably lots who never change the engine oil when their supposed to either.
Coolant is also supposed to be changed every couple of years. I think it becomes acidic after a while.

I really do not think there is any way for there to be rust inside the rear axle, unless you submerged it while fording a stream. Even if the axle and driveshaft where at the water level, the vent tube should be 12" higher, before it will take in water.

Still you should be changing the rear axle fluid every 25,000 miles on heavy duty vehicles, including a large truck. With synthetic fluid, you will not need to change it but every 50,000 miles or 10- 14 years.

Sure they will recommend a rear axle fluid change and new air filters every chance they can. They might tell you it will take 1 - 1.5 shop hours to remove the cover, drain the fluid, put back on a new gasket, and put everything back together, then add a couple of gallons of fluid.

Here is what I did to my F-350. I removed the drain plug. I bought a drill powered pump from Harbor Freight for $15. I bought a washing machine hose, with fittings on both ends, and cut it so one end was about 24" long the other went into a bucket. I pushed the short end into the fill hole, and ran the pump until no more fluid came out. This gets out most of it, say 85%. The fluid goes into both axle shafts, and will lube the bearings at the ends of the axle. The fluid in the tube helps to disperse all the heat generated while driving.

The axle can get to 250F when climbing a mountain, and if you happen to be putting a lot of HP to the tires, then go through a stream, the 200F air inside will shrink to 70F when it gets submerged in water, thus will take in a gulp of (hopefully) air. If it takes in water, it can change the oil to a white gunk, that will not lubricate the rear axle at all!

So for those reading this, if you ever get into water more than 24" deep, change the axle fluid within 100 miles or 1 week.

Good luck!

Fred.

From the 2001 Ford F53 chassis manual:

"Your vehicle's rear axle is filled with a synthetic rear axle lubricant and is considered lubricated for life. These lubricants do not need to be checked or changed unless a leak is suspected, service is required or the axle assembly has been submerged in water. The axle lubricant should be changed any time the rear axle has been submerged in water."

When I purchased my coach last year with no service records I changed the rear diff. Cheap insurance and personally I don't think many ever change it.

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.