I had no idea there were still 1/2 ton trucks with manual transfer cases! That's a good thing.
If it just is a part time transfer case (no 4 full time/ "automatic" or AWD options), it can use anything that is a light oil - safely. DexIII, ATF+4, expensive 8 speed auto transmission fluid ... etc.
But ATF+4 is cheap enough and probably what is called for