I replaced the doors on my '96 1500.
I originally got a set of quad cab doors CHEAP but they are entirely different than Reg cab/Ext cab doors. The reg cab doors are curved at the lower rear corner. The quad door is square. The rear of the door frame with the latch is completely different and there is no way, barring intricate metal surgery to interchange them.
The glass, guts and hinges are the same.
I got lucky and a salvage yard needed the quad doors so I traded even for a set of doors I could used.
BTW if someone wants a set of free reg cab doors that need work, let me know.