When dealing with electrical stuff. A moving fault normally means bad grounds or a bad/shorted out connector somewhere. I know you said you tested each part seperately. However, it might be time to start over and testing each compent by pulling the battery cables off the bats, undo them from where they go as well and all the wires off the starter. Pull the other end of the starter power cable at the strip on the firewall too.
Clean each and every connection. Use an Ohm meter to make sure each wire actually conducts from end to end. Put back on only the battery cables and the big wire at the starter motor. Leave the battery ground up by the headlights off until all this next step is done though. Get a momentary switch or remote start switch and clip it onto the leads at the starter. Basically, you want to connect the small post to the large post. Hook the ground cable to the battery and push the button.
Engine spins great then your starter is good, truck wiring bad.
Engine barely turns then your batteries could be bad, a single battery could be bad or the starter could need help.
Starter shakes, rattles and sounds like fingernails on a chalkboard but the engine doesn't turn could be bad Bendix drive on the starter or bad flywheel teeth.
Starter spins but nothing else happens means the solenoid is bad.
I know you are frustrated beyond belief right now. There is no great mystery to these trucks. Just a different collection of different parts than most people have ever worked on. They can still be isolated, tested and determined good or bad. Look at it this way. Once this experience is over. You will know the wiring system on your truck backward and forward, know you have a good starter and all the parts of the system are good. That is 30-100K miles worth of warm fuzzies right there.
I should also mention that the starters and alternators are the known fault points on these trucks. We had 5 CUCV's at the Austin Veterans Day parade yesterday. Once we were all staged and the parade started, one of the trucks didn't. We tried slaving it to another and figured out the starter had died. The truck was left, towed back after the parade and the owner swapped in a spare he thoughtfully carries in the back of the M1009. While he worked, the rest of us CUCV owners tossed tools to him and told our own "my starter died...." stories. Bank and food drive throughs seem to be common because we have to turn the trucks off to be heard there. Just part of the truck you either learn to live with, prepare for or run away from the trucks.
