RE: What Gear Oil to Use?
It all depends on what type of differential you have. If it is an open diff, then yes, you just pour gear lube right in until it is full. If it is a limited slip differential, you will likely need a friction modifier, or else you will have chatter, and eventually destroy your limited slip. Some limited slip differentials can get by without friction modifier as long as you use a synthetic gear lube. However others (like my Auburn unit) specifically call for non-synthetic hypoid gear fluid, and Ford or GM friction modifier. I just depends if your limited slip is a gear driven, or clutch packs, or cone style. Cone and clutch usually require friction modifier.
My '99 1500 calls for 80w-90 in the front Dana 44, and 75w-90 in the rear corp 9.25. However, my rear limited slip also calls for 80-w90, so that is what I use all around now. You can tell if you have a limited slip by looking at the tag in your glovebox or on the door (can't remeber which place says it). But if you are not the original owner, you should make sure a previous owner didn't install one. Jack up the rear end just enough to free turn the tires. If turning one side in a forward direction causes the opposite tire to turn in a backward direction, you have an open differential. If both turn in a forward direction, you have a limited slip.