No problem. I'm not sure if these heavy trucks had any partial VIN number stamped on the frame like the light duty trucks, but you might look around the frame rails towards the front and under the cab. I never really looked on my firetruck, might go check it tomorrow because I'm curious now lol.
sweetK30 Here's a link showing the VIN breakdown for your '88:
http://www.73-87chevytrucks.com/tech.../1988truck.pdf