According to the exploded view I saw of the transmission setup, it appears that there is a 2-speed hub clutch that uses a small pawl that pops out when it's spun fast enough and that pawl should engage the inside of the drum that covers the 2-speed hub. If the spring or pawl breaks in the hub, it won't shift and also if the "catch" in the 2-speed clutch drum is broken or damaged it won't shift. It appears, based on what the drawing showed, that the 2-speed works similar to that on the jato. The screw in the 2-speed hub controls the shift point - tighter screw = later shift, looser screw = earlier shift. If the spring under that screw is damaged, it may not tension the pawl correctly.

I'm just going on what the parts diagrams showed. I've never dealt with a center diff equipped truck.

The diagram I used: http://xtm-dealer.com/Mam%20explosion.html