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.
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Porsche or Country Coach!
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