Some food for thought.
1) I would first just check the starting system from the soleniod to the starter. Take the wiring harness and safety switches out of the picture.
a) Get a multimeter and check the voltage at the battery terminals. (Confirm you have 12.2-12.6 volts.) Keep in mind you have two batteries but one good batttery will not always be able to overcome a second bad battery.
b) Get a remote starter switch. (Auto parts store if you do not have one, or make one.) Hook one side to 12v positive of the battery and the other to the starter soleniod starter post. (Small post that sticks off the solenoid that the truck energizes with 12v positive when you turn your ignition switch to to start.) Ford has two posts, one energizes the soleniod to make the contacts connect inside (You want this one.), and one receives 12v positive when the soleniod energizes and makes contact. (For the distributor on gassers.)
c) Push the button and you should hear or feel the soleniod click. The starter should also turn over, if not you have an issue with the soleniod, the starter cable, the ground cable to the block, corrosion, or a bad starter but not necessarily the truck wiring harness. (You can leave your ignition switch off. It works either way in this test. This way the truck glow plugs do not vary the voltage and the truck will not start, just turn over.)
d) Watch the multimeter to see what the voltage is when you hit the starter switch. You should not go below 9 volts. A little drop but not turning over, likely the soleniod is not likely working. Big drop but no start, likely a bad starter. Your auto parts store can test the starterto verify its bad.
e) Now hook the multimeter or a test light to the wire from the wiring harness that energizes the soleniod. Check for 12v positive power in the ignition start position. (With clutch depressed and in nuetral.) If no power when the key is in the start position then you have/also have a wiring issue. (Broken wire, corrosion, bad connector, bad ignition switch, bad clutch safety switch, and not sure but if it has one a bad nuetral safety switch. (I have a automatic, so neutral safety, no clutch safety switch.))
Now you can figure out what you are chasing down to repair. The high amperage starter side or the wiring harness side of the soleniod.
Good luck!