I agree the 203 is a tuff one. upgrades I did to mine were the mile marker adjust a chain kit to keep it from excessive slop as they do wear/stretch start slipping/break & bust cases (DAMHIK!) the other mod was adding the mile marker part time kit in the 203 rear output differential housing. the full time bearings still turned the shaft/diff but it was not under any power in 2-hi

that's the other cool thing about the 203 is it has a differential built into it so it turns with out binding the front & rear can spin independent of each other (this why it does not drive off jack stands with 1 axle up- power goes out axle off ground) and when in hi-loc or low loc the front & rear output shafts are locked together = front does not turn without the rear turning (this will drive off stand with 1 axle up and has been a big safety problem when working on them)

but IMHO the w350 435/205-D60/D70 IMHO is the best bone stock & toughest units.

and yes the adapter for the 203/205 doubler is awesome behind the 435 even better is the nv4500 5 speed with the 205 or 203/205 doubler. awesome rock crawling and tuff as nails.

when I had my 79RC I swapped in the w350 running gear 360/435/205 D60-D70 4.56 lockers with a 6" lift & 17/40/15 ground Hawgs. only real issue I had was the short rear shaft and pinion angle. but rolling pinion up and a double cardan joint at each end of rear shaft fixed it.