Dana 50 from the '99+ era wasn't a bad axle. Don't lock it and bounce up the stairs at the park. IIRC plenty of 350's got stuck with them. Never say never, sometimes the factory makes weird combo's for dealer lot trucks. Even in todays computer age and just in time parts. Sometimes the ''just in time parts'' that arrive at the factory are wrong and get tossed on the wrong truck.
Frame's the same, brakes same. Some 350's have random weird option packs or different parts combos that might not mean anything. For example some years one had hydroboost brakes and the other vacuum.
Rear block is two inchs on 250, four on 350.
Some rarer max cargo option packs make the 250 exactly a 350, with a derate and a 250 badge.
If you're after slidein campers, you need to be careful about the 250's GVWR. Otherwise, both trucks are the same.