I don't have any experience with these trucks in particular in 4wd. Is it a manual shift or electric shift transfer case? I'm assuming electric shift. The pulse generator on other transfer cases typically looks at the speed of the output shafts. When the truck gets going over a certain speed (usually somewhere in the 50mph range) it will automatically disengage the transfer case and return it to 2wd. Another possibility is that it has something to do with the servo motor that actually shifts it in and out of 4wd, but in my experience that is typically located on the servo motor its self.