Make sure your linkage is working properly, and is shifting the mechanism completely into reverse. Disconnect the shifter from the transmission and see if the lever on the transmission will move further into reverse- the lever moves forward if I remember correctly. If the truck will move without jumping out of gear that way, you need to adjust the external shift levers so the internal mechanism moves the gears fully into mesh. The 1st/Reverse fork may also be worn, and not moving the sliding gear fully into reverse. Otherwise, worn/broken internal shaft bushings and bearings may be preventing the gears from engaging fully. Also, misalignment between the transmission case and bell housing will cause the transmission to jump out of gear, but in that case, the disengagement usually doesn't happen until you have driven a ways- maybe several miles.
Harvester
