I'd say a decent guide for normal folks would be 6 years or 60,000 miles. I think the schedule just says I for inspect. Really? how would you do that in the rear diff? just stick a finger in and see if oil is on it?
operation in the southwest, towing, racing, etc. would also be considered severe duty by many people - as would infrequent and short trips.
but really, a lot cars go to 150-200k and no one has changed that fluid. There's no combustion products in it, but, if you really use your soob on dirt, gravel roads and bad weather - where other cars would fear to venture out - then I think 30K or even 15 is not unreasonable. There is a vent and normal heating/cooling cycles will pull in a little dust and moisture over time. If you EVER ford deep water, you may need to swap the fluid asap as the cooling effect/pressure differential could pull water into the diff.