Ryan,
take the clutch cover off and look over on the right side of the chamber while you work the clutch lever to and fro. You should see a linkage that is supposed to be connected to the "other side" of the clutch yoke that goes rearward to the transmission interlock. It should be connected and moving and not flopping around at either end. If it is, then just hook it back up (pin is probably in the bottom of the clutch compartment or down the back of where the interlock rod goes. If it is all connected up, then take the transmission cover off (has the gear lever in the middle of it) and work the clutch lever again. The interlock runs across the three selector shafts sideways and has pins / springs that fit into the detents. See if you can move the selector shaft in question manually with the clutch disengaged / engine not running, and also try the other selector shafts. If the other ones can be put into gear, then the problem is either in the selector detents or the reverse gear is "stuck" on its shaft. Do you have the parts catalog? If so, it shows the layout.