Nomenclature. Most mechanics call the sealed, internally lubricated gearcase assembly that contains the differential, ring, and pinion an "axle." If it's a "solid axle" then the assembly will include the half shafts and possibly wheel bearings. Otherwise the half shafts and CV joints are outside the case, but the case is still the axle and contains axle fluid, which lubricates not only the diff but also the R&P set.
GM says I'm supposed to replace mine and I do.
The modern synthetic fluids last a very long time and with light to moderate use can easily last 100,000 miles or so which is as far out as the maintenance schedules go. All automakers are trying to minimize maintenance and have a goal of zero maintenance other than oil changes for the first 80,000-100,000 miles or so. Which is why we have dexcool. In that light the axle fluid is being dropped from the maintenance schedule in some cases where it perhaps should not be.
Usually the fluid requires replacement due to contamination before it fails due to shear. Water and grit from wearing parts are the two main contaminants. Limited slip diffs produce more grit. Water is more of a problem when there's high humidity and temperature extremes. There will be more wear and therefore more contamination and more shear if you work the axles hard. I live in Minnesota and have a plow on my truck and a gov-lock rear. So axle and transfer case fluid changes are a must for me. If you live in Arizona and don't play in the mud then maybe you can run the same fluid for 200,000 miles.
Jammer, excellent discussion. You taught me lots. Thanks-