Yep once you get some wear in the external shifter forks a prybar is a necessary item to carry lol, but usually it's because you are in two gears at once. On some different ones if they are wore, it helps if you twist the shifter one way or the other depending on which side of the H you are shifting. Kind of hard to explain on here, but the idea is to keep the end of the shifter from popping between the shifter forks. But it almost sounds like you got a different issue, you're stuck in only one gear. Not sure where the shrapnel come from, dunno, big chunks like that a never good to find.
Edit.....Thinking more about what I said, it is possible to have the shifter pop out between the forks and still only be in one gear, but when that happens you can tell something is screwy with the position of the shifter. Most often when you get into two gears is when you switch from one side of the H to the other. Just thought I'd clarify that.
Best I ever did was managed to get one into two gears at once roaring down the Yellowhead Trail in Edmonton when I was young working for a group of farmers, had unloaded at ATL (now Cargill) at about midnite, and was headed back to Redwater and trying to show off and stroking on it, grabbed a gear a bit too quick and managed to shove it into 3rd while still in second...got lucky, the driveline held as that old 960 skidded to a stop. I learned my lesson and kicked myself for my stupidity, but now it's kind of funny thinking back about it
I have a 72 GM tandem chassis with a good aux in it parted out. If you can't source one locally if it blows up or needs to be replaced drop me a pm with the model numbers off the tag on the tranny. If it matches won't be hard to make it yours.