Is it a manual or auto tranny? I'd start with the simplest possibilities first.

If it's a manual you should hear a clicking of the relay behind the passenger kick panel when you turn the key to start. If you're not hearing this check that the interlock switch on the clutch is working. Once in a while my floor mat gets under the clutch pedal and can prevent the switch from being activated. You can also check this by pressing the override button to the left of the steering wheel (truck in neutral <img src="/forums/images/graemlins/shiner.gif" alt="" />).

On the auto tranny I think there will be a switch that activated by the gear selector that will only let the truck start in park or neutral. That's where I'd start if you have an AT.