obvoiusly this is something that you have never done before. No problem, here's the basics. Once you've got the drain pan under the diff. housing and you're ready to get gear oil drippin' off your elbows, start by removing the bottom most diff. cover bolt. This way you know if one of the bolts goes all the way into the housing, thus making it a cover bolt/drain plug (chances are, NOT). If nothing comes out, remove all but the top few cover bolts, start prying the cover off at the bottom, a little at a time so you don't have oil all gushing out at once. When the bulk of the oil is out, remove the remaining cover bolts. When the cover is off, you may be surprised at just how clean it is in there. If you're not an experienced rear-end guy, (automobile rear-end, not the other) you probably don't want to be dinkin' around in there anyway so don't be takin' anything apart to clean it. As far as the cover goes, clean the inside of it, get rid of old gasket (on cover and housing) and look for any metal filings. If you see some, you need to ask someone else because I'm NOT a rear-end guy (automobile OR other) a new gasket should be available at your local parts store, if not, get some gasket material and make your own. Some guys just put a bead of silicone gasket maker on and call it good. Your fill plug should be on the side of the diff. housing, in front of the axle. It is a small plug about 1/2in. in diameter. It doesn't have a head on it so if it's covered in dirt you probably won't be able to see it. It should have a square hole in it that your 3/8 drive ratchet will fit into just like a socket. (man my fingers are gettin' sore!)
The plug should be about the same height as the drive shaft where it meets the diff. This is called the pinion. I just use 80W90 run of the mill gear oil. Fill the housing through this hole, (it takes patience) fill it until you can't fill any more. (up to the bottom of the fill hole) As far as your transmission, is it man. or auto.?
