If it's automatic, it's most likely got the 203, and I think the 81 and up got the 208.
If it's 4-speed it's most likely got the 205.
Look under the truck at the back of the transfer case, just under and to the right of the rear driveshaft yoke.
If it has a rounded cover that has three bolts holding it on, it's the NP-205.
If it has none but the case is made from cast iron, it is the NP-203.
If it has none and the case is made out of cast aluminum, it is the NP-208.
If it is none of those three, it "could" have the Dana 20, Rockwell, or Eaton transfer cases in it. All three of those were also available in GM's trucks, but mostly 1-ton and up for the Rockwell and Eaton units.
If you could post a picture fo the rear of the transfer case, there would be NO further question as to what you have.