Welcome to the bolt, Dune.
There were some factory flat beds with dual tires. The easiest way to tell is if the running boards end at the back of the cab. If the running boards look as if they were manufactured short, and not cut shorter, you likely have a factory setup.
--ps
I just caught the year of your truck - oops, I was thinking of the 55.1 and older ones. Someone more knowledgeable on your year truck will be able to answer more accurately than I. Sorry.
