Does it act like it wants to run as long as the key is in the cranking position but dies as soon as you release the key back to the run position? If so, check the wires going to the distributor to see if one has broken. One of those wires is a tach signal wire and if it breaks, the engine will act like it wants to run (or show an indicated spark) only while the engine is cranking. It might also help to know if this truck has a vacuum lockoff on the LP regulator or if it has an electric slow lock. Either type of regulator will have a big fuel inlet solenoid valve (down toward the rear of the regulator near fan) but only the late editions with TWC will have a small solenoid valve near the top front of the regulator.
Knowing which type of regulator lockoff will help in diagnosing as the two types are greatly different in their electrical circuits pertaining to ignition and fuel delivery.