The problem might be the switch-- the cylinder does not have anything to do with the ignition part. Pull the lower cover from the steering column. The ignition switch is right there--it is held in with a couple plastic tabs-- just pull it down. Then see if you can turn the cylinder.
The key cylinder turns a gear, and that turns a gear in the switch-- to turn ignition and starter on--or off.
The switch has a detent in the on position, so turn ther cylinder to "ON" when you insert the switch back in. That way it will be indexed correctly.
Chevy 04.5 LLY, 2X, shell, XDR, MikeL Heavy duty up-grade, 6spd, TransGo, deep pan, de-badged, 65 Gallon replacement Aero tank, LBZ-mp, Kennedy pump, shimmed, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags, custom grille, fender vents, HD trans cooler- Curt front hitch, footwell lights, load lights, Studs, head gaskets.--- Jerry