NP 205 if I remember correctly

 

It would be the NP205 and if remember, it would be the slip yoke model similiar to the 208.

 

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.

 

In 80, it could be any of the above. That was a transition year. Seems like the 80 models are different for some reason, but I don't recall what.

If your shift pattern is low loc-low-n-hi-hi loc, its a NP203.

If it's all aluminum case, it's a NP208.

If it's an iron case, w/ 4 low-n-2-hi-4 hi, it's a NP205.

I can't say I've ever seen one of the other unnits in a 3/4 ton. Just my experience.

I'd say NP 205 is the best all around, except for weight. I never had any confidence in a chain drive t-case, like the 203, 208.

Be sure you get one for hte same trans that you have. The splines are different between manual/automatic/TH400