Rear differential fluid. The roundish thing in the middle of your rear axle?

Very simple. Remove bottom bolts, drain fluid into pan, remove top bolts and take off the cover, scrape off old gasket, clean old fluid residue and crud out with CLEAN cloth, put a bead of new silicon gasket around the cover where it meets the differential, bolts it back up and fill with the correct amount of gear lube thru the hole...there is a little rubber thing plugging the hole up, just pop it out and fill thru the hole with the gear lube. Done. Takes about 30 mins.