Perhaps a broken spring that keeps whichever shift rail in gear.
I’d remove the shift lever (not the whole top yet) and see how it looks in there, even use a big screwdriver and work each shift rail forward and back to see how they feel. Might help to jack up one wheel and have someone slowly turn a wheel while you fiddle with the transmission.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-