From the new facts, the second sentence in the original post doesn't quite add up. It should maybe say the clutch doesn't disengage? That would line up with "stuck in gear", and with Wolfman's point that the shift fork could be broken (failing to disengage the clutch).
It might be possible now to move the high-low lever into neutral to move the machine to a better place for working on it (or winch it onto a flatbed or trailer).
With you in the seat, and with the Hi-Lo in NEUTRAL it might be possible to start the engine. You could, with the trans in 1st gear, and with the engine idling, push the Hi-Lo lever into LOW and creep the tractor along. Reverse would work this way too.
At idle it will be slow moving, and controllable by shutting it off or shifting the Hi-Lo out of gear. BE SAFE AND CAREFUL - don't allow anybody else near it when running.