Man if you have cng'd the brushes b/4, you're 3/4th of the way there. Tear into her and see what you find. Ain't rocket surgery.

Clockwork spring is what moves the shift fork. Motor turns the worm gear-> worm gear drives the "big gear"-> big gear winds up the clockwork spring (like winding up a clock)-> the other end of the spring is attached to the "little gear"-> little gear moves the shift fork. The reason for the spring is if the splines arn't lined up the spring will hold tension on the fork until they are and then it will lock. Reason I'm telling you is 'cus if any thing along the drive train is jammed it could cause a motor failure as well.

[edit]Then again if you were using a batt to turn the motor, you bypassed the limiting switches and may have caused an over travel condition. I still think you could fix it, common where's you sense of adventure <img src="/forums/images/graemlins/lol.gif" alt="" />