I haven't used a haybine in years but here's a couple of thoughts. Can you move the bar by hand fairly easily? Are all of the guards straight, any sections bent? Any chance you may have bent the cutterhead where the guards bolt on? Hit any groundhog holes, etc? I would take a shot in the dark and say your problems are related to excessive resistance from the cutterbar and not the gearbox. When the gearbox is disconnected from the bar, does it turn freely? Just some thoughts.
cfpinz