[OrrieG]

Not sure about GMC but the Chevys have the vin stamped on the top of the frame top rail just in front of the steering box and under the driver step. If the truck has been restored before the one in front of the box may be painted over. The one under the step can be cleaned and a transfer made using paper and lead pencil to get an image. The one under the cab was supposed to be a secret known only to law enforcement for vehicle recovery. All GM cars have a second one too, on my Chevelle it is on the back top of the rear passenger inner fender where you can see it from inside the trunk.