1989 F800 with Allison transmission; Chelsea 220 PTO won't engage

The truck itself serves as a fuel truck, which is where the problem started:

After fueling a plane, I disengaged (it's a Chelsea locking air switch) the PTO, heard the driveshaft stop turning ... but the light didn't go out, and the truck wouldn't go into gear. After a few failed re-tries to disengage, the ramp crew had to push me out of the way. I turned the truck off then restarted, and the light stayed off and the truck would go into gear. That's the pattern now - I go through the procedure to engage the PTO, the light comes on, but nothing physical actually happens. The PTO doesn't start turning, but I can't put the truck into gear - the PTO's engaged as far as the interlocks are concerned, and after toggling it off the light stays on and it won't go into gear unless I restart the truck. Which suggests electrical rather than mechanical?

After checking a few other things, I pulled the solenoid off the PTO, and everything looks sound, but I have no expertise at this. The fork moves by hand, but that doesn't mean it moves by air of course. And the middle input gear looks like it sticks in too far for the output gear to mesh with. I have no documentation/diagrams/manuals to work from to see if everything's aligned right, so I've included a bunch of pictures for hopefully far more expert eyes to see and tell me if anything jumps out.

EDIT: I marked the input gear with a grease pen then drove the truck a few dozen feet. The gear didn't move.

 

While I had the solenoid pulled, I marked the input gear with grease and ran the engine and even drove a loop... and the gear didn't move. I'm wondering if the dedicated PTO gear on the transmission itself is disengaged somehow, but I thought it was supposed to be full-time no matter what, as is the input gear on the PTO as a result. So even if the fork is shifting over properly, there's nothing moving to engage with.

Except that still wouldn't explain why turning the truck off and then restarting completely resets everything ... there's got to be an electrical malfunction, and I don't have the information on the AT-545 I need to know what sensors it has.