From your post, it sounds like the diff is installed as well as the drain and fill plugs. If so, then simply remove the fill plug to fill the oil in there and you are set. The comment above was related to a recent thread where someone removed the drain plug in order to change oil. All fine and dandy until they found the fill plug was stuck. So IF you are changing the oil, be sure to remove the FILL plug first then the DRAIN plug last. This way, you'll be able to easily refill the oil you drain out.