Stuck in gear sounds like a bent shift fork or maybe jammed up shift linkage. The only way to find the problem for sure is to start taking the motor apart. Start with the side cover opposite the shift lever, the shifter shaft runs across the motor, you will see the shift ratchet behind the side cover. If everything there looks ok, you will need to get inside the cases. Unfortunately, this means complete disassembly of the motor. Good luck.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.