Why is truck wanting to die when idling in gear?

What is your question?

Runs fantastic in park/neutral. Runs fantastic when on the gas. Wants to die when idling in gear, gets worse when truck is warmed up. Any Ideas. Truck sat for a couple years, I did put some sea foam in it and have ran out roughly 1/4 a tank of gas. Plan on running the tank empty and then putting some premium in w/fuel injector cleaner to hope this helps...but anyone got any ideas where to start if that doesn't work??