It's probably not any of those things.
You will need to get under the rig and look at the gears. It's likely you have broken a shear pin at the gear. This is basically a special bolt that secures the small gear assembly to the shaft. You can replace it with a regular 1/4" bolt but that wont be a true shear pin and will no longer protect the rack and pinion. This pin is designed to break if the slide is jambed.
Your vintage also had a ratcheting safety system that you could have worn out. It looks like a long, brass colored bell laying on its side.
Here's what it looks like.
At the 9:00 position, you can see the bolt on the other side of the metal plate (bolt is over exposed, sorry). It will likely be broken at one end, allowing the shaft to turn slightly in the hollow square tube as it moves.