Alright new belt came in- I compared it to the old one- it's the same size
I think it's definitely the tensioner- when PTO is running I can visually see the tensioner moving violently back and forth. I don't see how the tensioner could "go bad" though, it's just a torsion spring and there is resistance if I push on it by hand- but maybe it got weak and is no longer torquing the pulley. I read the manual to the mower deck and there really isn't much about it, or if I need to wind the tension spring around one more time.
So then I pivoted the assembly forwards- and I may be closer to the issue. It looks like some components are bent. Maybe I hit something like a wall when I was mowing before- the pulleys look like they are cocked sideways. Are the pulleys supposed to look like this? It looks like some parts are bent but I'm not sure if that's how they should be. I'll stick them in my vice and bend them back if not.
Thanks,
Eric