If you decide to try this yourself, let me give you one piece of advice. Every diff I've seen has a fill hole and a drain hole. Make sure you can open the fill hole before you open the drain hole.

You do NOT want to find out that you can't open the fill hole AFTER you've drained the diff.

Edit to add: It's an easy job and you can save a lot of money by doing it yourself, but good lord used diff oil stinks like you won't believe.