Ford: 1976 Ford F250 popping out of 4x4

Question
I have a '76 F250 Highboy 4x4. It's an automatic w/ manual locking hubs. 390
I live on the side of a mountain. When I lock it in 4x4 at the hubs, then shift it into 4x4 (I know it's full-time 4x4, but can go to 4x4 hi/low where all the tires spin at the same speed)...when I start climbing the mountain, it "POPS" and comes out of 4x4. I can sometimes forceably hold the shifter and it'll stay in 4x4 up the hill, but sometimes, even then, it pops out. It will consistently make the "popping" noise as I go up, and sometimes while coming back down.
I've had 2 people try to adjust the linkage, but hasn't worked. Any suggestions? Please help, I'm tearing up my driveway and probably the truck too!!!

Answer
Hate to be the bearer of bad news but apparently the locking ring inside the transfer case is worn. In that one, I believe it's like a ring with a set of ball bearings that the shift collar rides over when you shift it. Obviously worn and not holding. eBay is probably your best bet. Sorry I don't have better news. Good luck on your mountain.