Your original 5 speed trans was a M47 and was not too good of a unit unless you changed the type F automatic trans. fluid twice a year or more often with lots of driving.Their junk.
I'm not sure what 4 speed with o/d they installed but what you describe is the stick shift arm coming out of the trans from a very large nut that secures that stick into the trans. that vibrated loose. It's a fine thread large nut. But first you need to place the stick in through both shifter forks inside the trans then carefully tighten the fine thread nut.

Hopefully, the trans they installed is a M45 or M46 trans both are quite robust and can take a good beatin'.

Duck