[1969 GMC]

the 205 is probably best because it is so common, and parts availability is good. the only downfall is the relatively anemic low range (1.96:1) which doesn't really matter THAT much. there are a lot of other good transfer cases out there, but not well known. I would venture to say the Rockwell T-221 and the NP205 equal each other in strength, the Rockwell only lacking because of limited parts availability. It is also an all gear drive t-case, has a similar weak low range (1.94:1), but offers one advantage. the front and rear outputs are on the same level, and lower than the input, therefore easing the driveshaft angles a bit.
It depends what year truck it is for the transfer case. The Rockwell was used from 67-69, and also earlier in the 60-66 body style, but I don't know when they started using it. It could be either divorced or married, I am not sure what years it differentiated. My '69 has a Rockwell married to an SM465, most Rockwells came with the older SM420. All 205's came with the SM465. Here is a pic of my Rockwell, the easiest way to ID it is the quatrefoil (funny shaped) cover on the back, driver side of it.