RE: Automatic Transfer Case Fluid (GM Part No. 12378396) - what is?
It's pretty self explanitory... It's Automatic Transfer Case fluid... It has an additive for those transfer cases which allow for automatic engagement of the 4wd (NVG 233 - NP1 or NVG 136 - NP4). So if you do not have the auto 4wd option (NVG 236), you can just use the same ATF fluid as used in your transmission although GM recommends using the AutoTrak II fluid in all transfer cases. I am unaware of any alternative to it, but its not like the stuff is wickedly expensive.