Jon- How does the battery look? is it the original sealed AC Delco? Check the battery connections and grounds for good contact. It's possible the battery is getting weak, but the alternator is picking up the slack once the truck is running. Kinda strange tho- these trucks are so dependant on computer/electronics that they usually do all kinds of strange things when the voltage is low: Funky gauge readings & dash lights, tranny wont shift properly, etc..
G/luck
Joel
G/luck
Joel