I went to put up my drivers window today and about halfway heard a "pop", then it continued to go up crooked:

I can grab the window, and push it back so its straight again, its pivoting on something now. After I push it back into position it looks like this:

When it rolls down, it goes down fine and straight. When I roll it back up, I have to guide it manually so it goes straight.
Before I rip my door apart just to find out what part this is, then have to rip it apart again in a week when I get the part, I thought I'd ask and see if it was obvious to anyone what part is to blame. What do you think?