I'm also thinking that my derailleur hanger might be slightly bent, and have been contemplating purchasing the Park tool...but nearly $60 for a tool that is very seldom used is not easy to justify.

A couple months ago my commuter (Trek FX 7.5 - 9sp) fell over on it's right side. Ever since, my shifts haven't been as smooth, and I can't use the second-to-largest rear cog, as the chain is constantly trying to jump off of it - especially on hill climbs. I've adjusted the cable tension / barrel nuts with no success. All other gears work fine, the chain just won't stay on the second-to-largest cog.