If you have actually rusted enough to cause a hole in your axle housing, your frame is probably toast too. My best guess is you just need to remove the fill plug and put a fresh crush washer on there and reinstall the plug. It's suggested that you put fresh crush washers on every time you drain and refill your differential gear oil. A lot of people don't know this and reuse the old washer. With everything on our rigs, engine drain plug, transmission drain plug, transfer case fill and drain plugs, and differential fill and drain plugs, ALWAYS use new washers when you service them. The factory service manual recommends this for a reason.