More information would be helpful such as number of hours on your machine.
Assuming it has lower hours, one could exclude excessive internal clearances, due to wear but would want to look at shifter linkage.
Assuming high hours, again look at linkage, engine/transmission mounts, pilot bearing and of course internal wear if the other items are all correct.
I had a 1953 Willis Overland with three on the column. Second gear was up and it would want to jump out while going down steep hills using the engine as a brake. I rigged a bungee cord off the headliner and down around the shifter to keep it in 2nd gear. If you have a worn transmission, you could rig a similar fix.......
Tool Kit: Duct Tape, Bungee cords, Crescent Wrench, Lock Wire and a Hammer
