Got this figured out last night. The EGR valve was stuck open at 55%. My Actron scan tool showed the actual vs commanded EGR position and while the truck was being stupid, I could see the ECM was commanding 0% open, but the valve was actually stuck around 55%. A few key cycles later, and a few light hammer taps on the EGR valve, it free'd up and starting moving to commanded position. Instantly the truck runs flawlessly again. New EGR valve ordered.
2007 Classic Chevy 2500HD LBZ LT CCSB