I bought a Ford 2000, built in 1963, in about 1985. It was a 4 speed with a shuttle splined collar that upon activation, a collar was moved such that it contacted both the drive spline and the differential driven spline.
In road gear it would pop out. I changed the fluids in the purchasing process and when in road gear, would put my foot against the shifter lever, forcing it to stay in drive. Didn't take lone and it quit popping out of gear.............