The manual 4x4 mechanism is one of those "use it or lose it" items; if you don't engage it every now and then, it rusts into place. Slide under the truck. I'll bet you'll see a lot of rust on the linkage. Anoint it with PB-Blaster and see if she frees up. Thre's a write-up somewhere, where a fella put zerk fittings on the linkage to keep it greased. Don't remember if it was here or on one of the other PSD forums.
The linkage is replaceable; there's a Dorman aftermarket product, but it's crazy expensive (like $200-ish) for what you get.
My first car was a '72 Nova. PowerGlide. Weird coincidence, my 3 best friends all had '72 Novas, this in the early '80s in a Ford town (Dearborn).