The 4WD in my 2005 Blazer would not engage. I discovered a vacuum hose that had broken off that must be the culprit. I can't find the hose that plugs into this nipple though. You would think it would be hanging loose right below where it broke, but it is not. Can someone tell me what the other end of this hose links up to so I can follow it back? Here are two pictures for clarity:

 

Im going to take a guess and say it came of the actuator on top of the Tcase.Good luck with it.

 

It broke, but right at the nipple.

 

Fixed it. The actuator on the 4-button 4WD models is under the battery. Took off the battery and the platform it's on and found the actuator. Traced the vacuum line back, it had fallen behind and below the distributor box when it broke. Problem solved.