Ok, time for my experience on this tranny lock up problem. Had the exact same symptoms on my '54 Chevy 1/2 ton, 3sp on the column. When we took the truck off of the trailer we put it in gear and the truck must have rolled a little, putting pressure on the internal components of the transmission. A week of inspecting things and trying to work the gear shift out of gear had no results. So, Alvin(Achipmunk as he is known here) suggested that I jack up the rear end of the truck, set it on jack stands(so that it doesn't fall, and chock the wheels. With that done, either turn over the engine with the starter or turn it over by hand using the crankshaft pulley on the engine. In 10 minutes after getting the rear end off the ground my truck would go into all 4 gears with no problem. If you're engine isn't stuck, this should work to figure out if it is really locked up or not. If your engine is locked up, you'll need to remove the spark plugs, get some diesel fuel or Marvel Mystery Oil, maybe even some PB Blaster, pour some into all cylinders, let it set for a week or so, and then crawl underneath the truck, remove the flywheel cover, and gently pry on the flywheel teeth with about a 12" flat pry bar. After about a week of setting I barely had to push on the bar to turn the engine over. That's how I did mine anyway. Also, it will be a good idea to change the oil because it probably needs changed from setting and some of the stuff you put in the cylinders to free up the engine may have seeped past the rings into the oil pan. All of the necessary materials only cost $20, compared to who knows how much you'll spend on a new engine or transmission if you end up changing them out.
I hope this works for you if you try it. I'm 14 and got it done so just about anyone should be able to.
