It's a 4.8 liter.
Well ,I tried to solve this for 3 days in our spare time and we decided he should bring it to the dealer.

I found a leak right after the MAF sensor, fixed that and it ran better but still missfired at idle. The truck drives fine otherwise just misses and throws a lean code at idle. We threw new factory plugs and wires in it for good measure, I knew that wouldn't solve anything but we where just trying to eliminate that.

After that I sprayed starting fluid all around seams and the intake gasket area and heard no change in RPM.
PVC system is similar to a crossfire, just an oriface no rattle ball, that looked good.
Took off the throttle body and the inside of the manifold are seriously caked with oil and crud but that might be normal for a 100000 mile chevy.

Internet research pointed towards a stuck EGR valve open at idle (that would make sense), but I couldn't find one or any EGR plumbing. Maybe Silverado's don't have one.

ST Fuel trims at Idle are over 30%, as soon as you give it a little gas or drive the truck they go to normal (+/- 5%).

I'm stumped!