[truckster]

To get into the diff, put a pan under the housing and take out all the bolts on the cover. It will probably start leaking 90w as soon as you take out the bottom bolts. Use a new gasket to put the cover back on (some people use silicone, but I like using a gasket), and make sure you put the cover back on in the same position (it's really easy to get them upside-down if you've got an Eaton). Once you've got it bolted up, just pull the access plug (it's about halfway up on the cover and has either a square 1/2 in recess or a square knob on it) and fill it with 90w until it reaches the bottom of the plug hole. If you find you have a posi in there, you'll also need a friction modifier additive from GM.