New guy here but have info that may help. A few years ago I hit a deer with my 2005 Express 3500 (6L). Shop fixed damage and van did great for three days. After much head scratching we found a module underhood in front of steering wheel that had one busted clip, the remaining clip held well enough for intermittent contact. This caused the van to run on 4 cylinders as to prompt me to get it to a shop. Mech showed me a GM bulletin about moisture getting into this connection. May be worth a look.