[steelerguy]

Every 25,000 miles is what I have in my manual. Make sure you add the slip differential treatment. It's about 2.5 qts of 75/90W (per my Haynes manual) diff oil at $4 or $5 a Quart. The additive is about $7. If you have 4WD, make sure you do the front as well if you haven't already. Will also need some high temp RTV (they don't make seals) & a torque wrench. Mine is 30 ft/lbs & I also clean the entire assembly with brake cleaner. Takes about an hour.

The rear is easy. Just have to drop your spare tire out of the way. The front is slower because it's not as accessible. Takes a little less less oil & I add the slip diff additive because I was told it doesn't hurt for it to be in there even if it's not required.

Get a $12 Haynes manual at autozone for pretty good instructions.