I've read here that the differential cover can't be removed without dropping the entire differential down a little bit. If I were in your shoes, I would remove the drain plug at the bottom (looks like a square hole that a ratchet fits into). Once the fluid is drained out, I'd loosen all the bolts holding the cover on and completely remove the one with the damage around it and squeeze a butt-load of permatex all around that area and tighten up all the bolts, fill the fluid and hit the road.