GMC Troubleshooting Questions? Ask a Mechanic for Answers
It sounds like you took care of most of the causes of a stumble or misfire already. At this point how is the truck running? Are you getting any more trouble codes?
It sort of sounds like you may have a couple of dead lifters. The lifters on these engines tend to go bad fairly often. It's hard to say without more info though.
I think we'll need the trouble codes to know what's going on.