[JThiessen]

Sounds to me like you messed up something while reinstalling the gears.
Personally, I would buy the G-maxx single speed kit and cover plate from Mike and use that tranny.

If you do go with the 3905 tranny, you'll have to also change the center shafts (#1 due to the length difference, #2, It uses Revo style ends). Its definitley a fairly easy conversion, but might be more work than you are looking for. On the plus side, you would get a better slipper system than the 3906 has.