What I would do is go back and check every step you took replacing the drive gear. It didn't before but it does now sounds very much like something is not connected right. Check every wire you moved or removed.

I went back and checked everything. Nothing seems to be out of place. What would cause the unit to turn on as soon as it gets plugged in? Also, when it is plugged in, it doesn't respond to any of the remotes, switches, sensors, etc. What should i do to stop this?

The only thing I can think of is if the logic board is not plugged in correctly. This shouldn't happen though as it is made to plug in one way only.

I would be interested to know if the smaller gear on the end of shaft of the gear you replaced is turning the limit switch controls.

I checked and the smaller gear is not turning the limit controls. It acts like it's slipping. I'll have the smaller gear lined up, but when i plug the unit in the motors starts running and the shaft kind of pushes the small gear away and keeps it from lining up.

I have tried turning the smaller gear by hand to get the limit terminals to touch and the motor still doesn't respond.

My guess, is that it has to be something with the smaller gear though, as nothing else seems to be out of place.

I have seen this before. On the main gear drive right below the sprocket where the chain goes around, there is a bushing that ceters the main gear drive and that is probably worn out. You may notice a little bit of slack in the chain too. This is caused by normal wear and tear but can accelerated by tightening the chain too much.

Good luck