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@SpecialWarr Hijack accepted. So, as long as the WD21 differential is also an LSD unit, then you should be able to swap the side gears over. You'll likely want to bring over the spider gears as well, but can omit the thrust block (if one). This would basically make your WD21 diff support 33-splines. If we're not talking LSD diffs, then no, the side gears can't be swapped, but the entire diff might be able to if the WD21 gears use 12mm bolts...I think it was tied to the use of 11 attachment studs on the housing vs. 9 studs. I'm not sure what the cut-off was between 11- and 9-stud, might've been 1989 or maybe didn't exist on WD21 and was only a D21. I'm honestly not sure. FWIW, I've had no problems finding replacement H233B parts from RockAuto. In fact, pretty much the only thing I get OEM are the gaskets (38320-T3322). Not that there are many parts to replace in these diffs, but I've used National 32306C (pinion bearing & race), 32308C (pinion bearing & race), and 710195 (pinion seal) before. Can't seem to find any differential carrier bearings I've purchased, but OE p/n is 38440-60000 which cross-refs to National KC11445Y. Needless to say, these parts seem readily available so not sure what that mechanic is smoking. As for "what else"...probably not much more. There are oil seals at the ends of the axle tube that can be changed. Only way to replace the grease seals are to tear down the axle shafts. Beyond that, new gasket and fluid.