Be CERTAIN to remove the fill plug first. Nothing worse than letting all the oi out of the diff. only to find that the fill plug is frozen. If you are concerned about buildup in the diff., you might consider running it for a few hundred to a thousand miles with fresh Mobil 1. Then drain it. The condition of the oil coming out will let you know if you need to lather, rinse and repeat.
The diff. requires a bit less than 2 qts.