If there were no problems before it has to be an assembly problem. Suggest studying manual/videos before disassembly, might catch something on breakdown that wasn't quite right. Don't know if shift forks can even go in the wrong place, never had that problem.
By chance did you heat the cases for any reason? I've had bearings fall out during this process and once had one move just enough to cause binding. Was tearing my hair out much as you are.
First two times had sealant on cases. Second time around was checking shafts, everything was good. Torquing it down shafts bound up. Then I assembled bare, one shaft at a time. Right side output shaft bearing was a hair to deep. After adjustment everything was fine.
Might have to do same as me, one shaft at a time including shift drum, looking for problem.