pull the cover off of the diff to drain it and on the pass side of the pumpkin there will be a little plug that a 3/8 ratchet will fit into. pull the plug out and fill untill oil comes out of the fill hole and that is it.
I have 2 different gear lube pumps (kinda like the pumps on a bottle of "Go-Jo" hand cleaner) that you screw into the bottle and use a hose to pump fluid up to the diff or transmission. Try your local parts store and ask for a "gear lube pump". I bet they can fix you up for under $10.