Window?

I would suspect you have warn windows pushings. 50resto sells replacements. You'll need 2 per side for a HB/notch or 1 per side for a vert. They encumpass the guide rail that the window travels up and down on. I've done an initial writeup on how to replace them. I should have something more thorough (and with pictures) in a couple of weeks, since I have to do the other side.

The first time you do it it may take a couple hours. My friend and I had no idea what we were getting in to when we first did it and it took maybe 3 hours. The next time took maybe an hour.