Tried Simple Green and it just couldn't cut the really thick, caked on grease.

Got some "safer" Klean-Strip Green Paint Thinner (milky-white kind) and that did the trick no problem. I tried to avoid plastic parts at first but ended up just soaking everything in it and scrubbing with a toothbrush. It melted my latex gloves but the composite diff housing was very resistant to this solvent and I don't notice any damage after using it (not to say the resin isn't compromised somehow though???).

After switching to Nitrile gloves I cleaned up the rest of the diff and it looks like new. The silicone/rubber O-rings were briefly exposed to the paint thinner but I washed em off quick with water and they seem fine...might order replacement O-rings just to be safe...

I would recommend this type of paint thinner for anyone who wants to do this but be aware that it will melt certain plastics but seems to do less damage to other hardier plastics...?

Maybe someone can comment on what is going on here with the plastics and paint thinner.