I would first look for any issues external to the transmission, namely the bushings where the shift levers connect to the arms on the transmission. They wear out after awhile. Also check the shift lever adjustment at the lower end of the steering column. Place the lever at the top of the column in the neutral position, it should be horizontal or pretty close to horizontal. Then look at the tops of the 2 shift levers at the bottom of the column; they should be on the same plane relative to each other. If they aren't, disconnect them from the column and adjust the clevises.

https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.dennis-carpenter.com%2Ftrucks%2Ftransmission%2Fgear-shifter%2Fc5tz-7343-a-column-gear-shift-repair-kit&psig=AOvVaw0GxCq9G1i3Jvc4_GrLCC7m&ust=1612038691451000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCJDi_6n-we4CFQAAAAAdAAAAABAD

Internally, I would suspect the second gear synchronizer is getting worn out. or the 2nd gear bushing may have too much clearance on the main shaft which could be either gear bushing or shaft wear.

Here is a YouTube video on rebuilding the Borg-Warner T-86 with OD. 

https://www.youtube.com/watch?v=o2tFwBTrLRQ

I have a issuse that am trying to get as much to know before I dig into the work.  My 3 spd OD which is a top loader in my 55 Ford seem to want to drop out of 2nd gear and go to natural at time.  So before I tear into it need as much insight as I can get to what to look at or for.  I know that under the cover there's a spring with a small ball bearing. Does someone know what this does?  Inside there are shifts that are grooved for the ball to slide into as the forks move to different positions.  Would this be a area that might cause the dropping out of 2nd gear issuse? 
I kind of working of what left of my memory when I was much younger.  Back then I would just rip into it but now at my age i would like to know what to look for, as those old task have become much harder to deal with.
You all have seem to have great input on other issuse I ask about for that I say great job and great people.  HELP