Why would a 2004 truck start run at idle then die?

Could be any of several issues including but not limited to: clogged fuel filter, bad fuel pump (or perhaps a blown fuel pump fuse), or even something as simple as a vacuum leak. A bad fuel pump can sometimes supply enough pressure to provide the fuel needed to idle, but as soon as there is a load on the engine it is starved and cuts out. Same with a vacuum leak, it will idle fine until there is a load on the engine.

With the engine off (alot of moving parts you don't want to get caught up in) Look for a hose that seems to be disconnected. If you dont have or cant get a repair manual (haynes or chilton's) checkyour local library, many carry a wide selection of repair manuals in the reference section and they all include a troubleshooting section that may help as well as vacuum diagrams to help you chase down a vacuum leak.

If it is a Ford, there is an inertia switch that can be triggered by an accident or even a hard run in with a pothole. It shuts off the fuel to the engine. If that's the case, below your radio where the hump is in the middle of the floor is a black panel right along the carpet. If you remove that the switch is hidden behind there. Just push the red button down and it will reset.