Changing the gear lube is simple. Remove the ten differential cover bolts, pry the cover off with a screwdriver, allow the old lube to drain into a bucket, scrape the old RTV from the cover and differential housing, pull the fill plug from the cover, apply new RTV to the cover, reinstall the cover and bolts, fill the differential to the bottom of the fill hole, and reinstall the plug. Use a scraper or razor to remove the old RTV and run the new RTV bead around both sides of the bolt holes. Be sure to add LSD additive if you have a limited slip. Check out the beginning of this page:
http://www.fuzzymuzzle.com/Car/Gear_Ins ... r_8.25.htm and the Lube section of this page:
http://en.wikibooks.org/wiki/Jeep_Liber ... er_8.25%22