To test the operation of the starter you can bypass the ignition switch, nuetral saftey switch, and everything else by running a wire from the positive terminal of the battery. You take this and jump the terminal on the starter solenoid. This is the unit that is attatched to the starter. This should start the truck right up. If it doesnt, then you have a problem with: your grounds, your starter, or your solinoid.

Just becarefull to make contact at the solinoid, and not at the starter (the bigger terminal), if you jump the starter only you will not properly engage the bendix? and you can damadge you fly wheel or flex plate.