1979 GMC 4-speed linkage adjustments

Unfortunatly, what you are saying sounds like it's not the linkage adjustment but possiby a bad sychronizer in 2nd gear. Does it pop out running easily down the road or just when you are under accelleration? If you are out of adjustment on your linkage it could be pulling it out of gear but doesn't seem too likely. Have you tried to hold the shifter in gear? Does it "kick" the shifter back to nuetral even if you are holding it?
This would lean towards the gear sychronizer. Hate to be the bearer of bad news [ if I am] but check it out before you ruin 2nd all the way. Luck and lots of it! :Shock:

If it isn't engaging as fully as the other gears you might try adjusting the stop on your shifter. On a 4-speed it should be the front one towards the pass. Side Back it off intil you can feel it go into gear without having to apply pressure to make it go completely in. If it still jumps out at this point you will have to see wether it is the teeth on the gear or on synchro. What really bites is ussually the part isn't costly- it's the labor! But if you like the tranny that much the more it jumps out like that the more damage is likely to be done. Bypassing 2nd will work for a while as long as it doesn't put too much strain on your motor. Hope turning some nuts and bolts will do it for you! :P