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Question: How often should I change the differential fluid?

I bout my 2006 Montero used with 67K miles on it. It now has 82,600 on it. It was cherry when I bought it, no kidding- never driven off road, no tears in leather seats (there are now). II am off road in muddy or dusty conditions about once a month for a couple of days. The rest is city driving, stop/go, wait. have added a K&N filter, OME 2" lift via struts/springs front and rear, heavy duty. I have added 285R75/17 ProComp Extreme A/T tires x 5 on stock rims. This suspension/tire setup is awesome, very stable with plenty of bite and clearance for the trails I'll be using. I hope to add ARB AirLockers front and rear within the next year. I've also scratched the rear bumper a bit, now have BlackHorse? bumper guard. Also want to add 2nd battery, on board air compressor, of course, for the lockers and a oad of other goodies, like lights, LED accents, rack w/tent, fire extinguisher, full medical kit (I'm an NP), fridge, good seat covers and a few other odds and ends. Should I just wait until I hit about 90,000 miles and do lockers and fluid change then?
Thanks,
John