K, here's what you do. Scrape of the major stuff with a plastic scraper. Then get some Easy Off Oven Cleaner-the lemon scented one is quite pleasant. Spray it on, scrubb a bit with a brush, and then rinse off.

The nice thing about the oven cleaner is that it becomes water soluble, so it's easy to rinse. If the diff is out of the car, I would tape some plastic over the drive flanges so that you don't get water in the bearings or gears.

When you get done it should look like this:

Thanks guys! Looks like I just need some spray cleaner and a wire brush for the worst of it. I'd like to pull the rear cover to see what gear ratio it is and just to look becuz I've never opened a differential before, so maybe I'll do the external wire brush scrub and then open it up and refill it with fresh oil and paint it so it's shiny like Kinfish's.

I used a powerwasher, then small bbq wire brush with solvent and repeat. I did scratch up the front aluminum with the brush when I tried it there so I used a softer nylon golf club cleaner bruss.