Agree with Sman....usually the best. Take 'em all apart (one at a time) remove any rubber o-rings (or any rubber products, including the bearings) and give everything a good soaking with carb cleaner or something like that. Put it all back together and fill them with the oil of your choice.

Note: Don't forget to put the rubber O-rings back and don't forget to put the grub screw back in before filling the diffs with oil......you may end up with a bit of a mess. Oh...and if they look like they need it, replace the gaskets. Better to do that now than find a nice thin line of diff oil all around the inside of your shell and chasie next time you run it.

Have fun man.