With some clean, clear tube from a hardware store, or if you have some laying around, you can fill the rear diff pretty easy.
It takes two people however.
1) Remove fill and drain plugs.
2) I like to wash out the housing with some brake cleaner, or carb cleaner. Just spray it in there, and let it wash out all the old oil/crap possible.
3) Let it air dry out any cleaner you have put in. (Open for 10 min. is all it takes.)
4) Put in the drain plug.
5) Route your clear hose to the fill port. Have your assistant standing with the gear lube container and the other end of the hose on the Right Rear wheel area. You will find the hose routes up by the tire/wheel just fine, and goes down from there to the fill port.
6) While you hold the hose in the diff, and monitor how much fluid is in there, your assistant inserts the clipped end of one pointed cap of gear lube, and GENTLY pours/squeezes the gear lube down the tube.
7) IIRC, when you have two full quarts in there, it only takes a little more.. So do not go hog wild, or your going to have gear lube all over the place. (Any extra lube in the hose will have to go somewhere.)
Or just buy a pump, or dispensor like Upgraded's (I used a pump for years, and the hose is just faster/easy in my opinion, but filling up the transmisison, the pump is simple.)