Ran across this VIN tool in a car forum I go to. Kinda cool to see the original build date and details about my truck. I did notice one error, I have a 30 gallon tank (I have original window sticker as well) and decoder said 24 gallons. Mine was built May 26, 1993, I would guess the last few months of first gen assembly.

http://www.compnine.com/vid.php

 

24-gallon tanks were the standard for all of the trucks. The diesel engine option automatically got you a 30-gallon tank. Your window sticker should list that under the options.