Do you know if the truck has any modifications to it such as a chip or tuner? You need to find out what Diagnostic Trouble Code (DTC) was set (if the CEL came on, a code was set). Most auto parts store readers won't read these trucks, but a decent mechanic should be able to do that for you. Alternatively, if you have a smartphone or laptop, you can download the Forscan (or Forescan Lite) app and along with an appropriate ELM-327 OBD-II Interface adapter, you could read the code(s). As an example, I use an OBDLink MX+ bbluetooth adapter.
In the interim, you can inspect the injector/glowplug wiring harness connections at the valvecovers. There are 2 on each valve cover. Look for evidence of burned/melted pins and oil in the connections. You can also check your fuel pressure at the schrader valve (tire-type valve) on the fuel pressure regulator attached to the drivers side of the fuel filter bowl on top of the engine. It should have around 45-60 psi at idle. You can use a cheapo stick-type tire gauge, but wrap a rag around the joint to avoid a fuel bath! Cheers!