[Dingfodgy]

I would check the connections at the battery, and fusible link first. Then I'd get under the truck and make sure the power and solenoid wire connections are clean and tight on the starter itself. Also check all the grounds (ie- body to frame, body to engine, engine to frame, etc.)
If all that looks good- get up under the dash and put a jumper between the two purple wires on the neutral safety switch and see if that might be the issue. If there is still no improvement- pull the ignition switch plug and run a jumper from the 12v feed connector to the ignition "on" connector and another from 12v feed that you'll just manually contact with the "start" connector and see if that works.