'51 Chevy PU - Keeps popping out of gear

I would believe that symptom indicates either or both broken/ excessively worn gears, or a faulty throwout bearing. Given the vinatge of your vehicle, and assuming the parts in question, even if not as old as the truck itself, are at least very old, I suspect clutch and/or transmission work, not mere shift linkage tinkering, would be in order.

Yep. This happened on my '53 chevy too, in 1965. It required new gears in the transmission. They were easier to find back then.

As I feared. I of course always prefer tinkering to replacement.

Getting the parts are quite easy, it's diving into a big project I feared! It may face a true mechanic on this one. I may just have to hold it in gear for a spell.
Thanks for the expertise!

There isn't much tinkering you can do, sometimes on the old aluminum case muncey the problem is the hole where the shift lever goes into the case gets worn and allows slop, you can look an see if this is the problem. If not is is worn inside the transmission.

Final Diagnosis
$1,300 later...the entire transmission needed to be rebuilt. Gears worn, bushings shot, etc. I'm surprised it ran as long as it did after seeing the case opened. Big bummer--but thanks for the heads up.