That's fairly typical sludge. You can wipe out all of the chunks with rags, and then do the final cleaning with brake cleaner. Considering how horrible that is, I'd be tempted to remove the rearend and remove the axles and axle tubes, as there's probably a quart and a half of nasty Hytran trapped in the axle housings. You'll need new gaskets for the axle tubes and new axle seals, but I think it'd be worth it, and you're halfway there already.
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