You also need to confirm that the check engine light in the upper right corner of the dash monitor is lit when the key is turned on, but before the engine is started, and that the light goes out when the engine is running.
If the light is not lit when the key is turned on before starting, the bulb is probably burned out from being lit too long.
None of that about the bulb is going to fix the running problem, but if the truck is having some issues like you describe, I would certainly expect the check engine light to be lit.......and you made no mention of that.
Yes, there is an injector underneath the mixer on this engine.
The injectors do become faulty with age, and mostly plug up with the heavy end byproducts of the LP fuel. I would expect this to be more of a problem if the engine is run most often on gasoline with only intermittent running on LPG.
Cleaning the injectors is usually an exercise in futility.
If you have confirmed that there is no LP being delivered by the injector, replace the injector.
The injector is "fed hot" from the TWC relay but the ECU controls the grounding of the injector via 2 pins on the ECU. The injector is activated by a "non resistor" pin first to assure the injector is activated and then the ground function is handed off to a pin on the ECU that enables the injector to be held in the activated position. This 2 step circuit scheme is to prevent overheating of the injector solenoid. High current to engage the injector with low current to keep it engaged.