The Stovebolt Site Forums The Shop Area Driveline Jumping out of gear

Unfortunately it's a fairly common event with 3 speed transmissions both in the early transmissions with a torque tube and the later open driveline version. I would start with the shift linkage and make sure it fully engages and doesn't have a lot of slop.
Next check the universal joint which can cause a vibration effect and cause it to jump out of gear. It could possibly be a worn 2-3 shift fork and/or weak springs on the shift fork detent balls. Worn bearings can also cause an issue similar to the u-joint problem. And of course the good old syncronizer assembly which is a common wear part.
Or it could be a combination of all of the above.
Fred