Well, I pulled the IAC and it looked pretty bad. It was gunked up to the point where I couldn't get it clean enough to satisfy me, so I replaced it. I'll know if this was the problem or not next cold start. I hope so!

I took a peek down inside the throttle body and, sure enough, there's an oily residue in there. I assume this means the plennum gasket thingy is leaking. Is there any harm in continuing to drive the truck before tackling that job? What is the harm?

 

Cold idle problem solved with the new IAC (who knows why just when it was cold??). Now to worry about the darn plenum gasket.