I'd take off the door panel and look at the track first. It might just be some weird bend in it that would throw it off track. At the very least, you could take off the panel and roll up the window and see where it comes off the track. That could be a start. I think the only way to have the door panel off and still have control over that window is to have the window switch still plugged into the door.

I had this problem a long time ago. Clean out the front and rear window tracks with wet paper towels. Just stuff it in there and rub it in. Haha, that sounded dirty. . Use strong paper towels so it won't break apart in there. Don't use rags or terry towels because they will leave lots of lint stuck in the tracks. Ok, then get some silicone lubricant in a spray can, use the straw, and spray it generously into the tracks. Run the window up and down, holding the back end down so it won't pop out Run it up and down a few times to spread it and wipe off any excess. , that kinda sounded dirty too. Sorry, but yeah, that should do it. Some cleaning and lots of silicone lube into the tracks, especially the rear one.