I'll try some PB blaster on it, clearly something is stuck! It is not dirty, corroded or in any way looks like it is more than a year old and always garaged.
I'd be worried about breaking the screw off also with a wrench, and I'd be worried about messing up the threads, something I'd prefer not to do since it works now, just not 100% where I think it should be. Mine does pull in first gear, but I run out of power to the wheels way before the engine bogs. Last night I loaded the back of the machine up with 9 oak wheels, and had my trailer on the back loaded with three 40+ inch wheels. We made it up some very steep hills here, but it was marginal at best.
I'll try to work on it this week end, but I need it to do work so I am bit concerned about breaking something and making it worse or unuseable.
regards
Wayne