use a 0000 steel wool to dust off the rusty areas.... then blow clean with an air compressor, spray liberally with WD40. rub the whole gearset as to polish with the wd useing your thumb/finger.... dry off with cloth and blow dry.. by rubbing the wd40 around, you help it seep into the pores of the metal... Old trick i got from Gramps..
get a Sealed Bearing Set for your diffs, they Typically have a Blue color to the bearing, though not always. a sealed bearing will stop water seepage and protect and extend bearing life...
hth
Timmahh