Suspect the neutral safety switch first. To start, bypass the starter relay to see if the starter will crank the engine. You can pop the cover off the relay, install it that way, then squeeze the contact, or you can use a piece of wire or stretched-out paper clip to jump the two terminals in the socket. If your truck uses the 1" cube relays like in my drawing, jump terminals 30 and 87. If you have a manual transmission, be sure it's out of gear. If you have an automatic, be sure it's in park. If the ignition switch is on, the engine will start.
If that works, you can try swapping in a different relay but the next suspect will be the cam on the end of the ignition switch cylinder.
Sunday, July 28th, 2013 AT 8:25 PM