One other thing: gear oil smells like a bag of dead a$$holes. Use latex gloves when getting in there and old clothes. Trust me on that one. The smell sticks.
he is 150% correct. also, due to the location of the fill hole on top of the rear end housing, and the fact that the GT dual exhaust runs right there on both sides, it is hard to not spill the crap everywhere when trying to fill up the diff. not much room to maneuver the oil bottle. if you have access to a fluid pump to pump the oil in, i would highly recommend it. otherwise take Yadkins advice on the tube. i got oil all over the damn exhaust trying to fit the bottle up in that super tight location and my car smelled like a cooked bag of dead a$$holes for a month.
other than that it is a piece of cake. put pan under diff cover, unbolt the diff cover, drain fluid, take some old but not dirty shop rags or some paper towels (buy a roll of the blue ones from oreillys or walmart in the auto section, not household towels) and wipe out the housing as good as you can.
cover the carrier and gears with either a plastic bag or some shop rags / blue towels to keep them clean from the next step. take a die grinder and clean up the mating surfaces on both the housing and diff cover. GET THEM BOTH AS CLEAN AS POSSIBLE, you dont want any leaks.
if you dont have a die grinder, buy one, they are not that expensive and have a ton of uses. scrapers will work too, but scraping sucks. if you are fortunate enough have a bench grinder with a wire wheel, that sucker will make quick work of cleaning up the diff cover. wipe out housing again from any debris that got in from this step.
you can also use a parts washer on the diff cover if you have one, just make sure you wipe all the solvent off afterwards. you can either use RTV or a new gasket (i prefer gaskets) to seal the diff cover back on. if you use RTV, get that cover on quick after you apply it to the housing. i didnt torque the bolts, just tightened them up good, but not too tight or they will strip and/or distort the cover, gasket, or RTV. then fill her up. oil hole is on top of housing, drivers side. fill it till it overflows out the top, then put in plug, tighten good but not too tight and you are good to go.