Also sounds to me like the pinion bearing. Had one go out in a friends 8" (basically the same design, only smaller scale). We ended up just replacing the entire chunk, swapping with someone who knew how to rebuild it properly. Kind of like a core exchange I guess, but without having to pay someone for the install labor.
From what I've seen, it's not too complicated of an assembly, but you have to have the right tools to do everything perfectly. Good luck with whatever you decide.