Do you have access to another SM-420? Maybe a parts trans? If so swap the top cover, making sure the forks are the same. Sounds like the problem is in the top cover on one of the shift rails. I work part time with my wifes uncle at his garage. He works mainly on older medium duty grain trucks and I have seen this issue before. Usually he will swap the cover out and repair the old cover at a later date. I have seen both the Sm-420 and SM-465 get stuck like this. The trucks usually show up on a hook. I am not sure how he fixes them but I think that there is a spring and ball type detent that gets stuck on the shaft that supports the forks. Sometimes the fork will get bent and the shaft gets damaged from forcing the truck in or out of gear. Good luck. Patience endureth all things.
