just make sure you move it with the truck on and runnin but with it in nuetral, and when you do it, dont stop the xfer case in nuetral i know it says it has one, but you dont wanna do it trust me. just one solid fluid motion forward or back to get you into the 4wd setting you want.
the CAD has 2 vac lines, one should have vacuum when in 2wd, and then when you shift it all into 4wd, it should swap vacuum to the other line. if it doesnt do this then you know its the vacuum. easy place to start, and 90% of the time its that. just unhook the 2 lines at the CAD and put ur finger on them and feel for it. sometimes they break and crack, or get clogged. you can clean them and cut off the bad section and splice in a piece of hose or what not to get it going again.