Bill,
Could the garage have let the truck sit and idle for an extended period of time in an effort to check something.
Maybe they lost track of it and it overheated just sitting, you showed up and got the results of there mistake.
If it cooled off as you drove, the cooling system is operating properly. I would fill the radiator to the proper level with your anti-freeze cocktail, take it for a drive that will bring it up to temp and verify all is well before I put it in mothballs for the winter.
My guess is that all is fine.
Stuart
