I'd normally go with "check your manual"- until we opened up my JK (6000 miles) for a gear change (4.56s). My diff. fliud in the rear was a mess- sludge and metal fragments stuck on the magnet. My pinion shaft bearings were frozen onto the shaft. Don't know what came first- the chicken or the egg. (The metal caused the bearing to seize- or the bearing seized and caused metal fragments- or neither). I'd suggest it might not be a bad idea to check your diff. fluids early. It may never have caused any damage- but I wouldn't want to chance it.
(My front. fluid was fine- and the pinion shaft popped right out.)

 

This guy IS lost! $200, 7 bottles of differential fluid, this guy is nuts! It barely takes one bottle for the front diff, barely two for the rear. Most synthetics have LSD additives and will meet Chrysler's API GL-5 standards, not voiding your warranty! Look at the owners manual, it says in black and white "Mopar Gear and Axle Lube or equivalent". Valvoline synthetic will only set you back $10 a bottle, $30 total, don't listen to this lost hype.

 

I am certain that I am in the minority.....but I constantly have my fluids changed.

Radiator: Every 2 years
Transfer Case: Annually
Transmission: Annually
Diffs: Annually
Oil: Every 3k
Brakes: Annually
Steering: Annually
Rotations: Every Oil Change

I luckily get a nice discount from work.....so it is a fair price to drop the vehicle at one of our stores and have them do the fluids. If I had to pay shop rates, I would do it myself.