Does the Climate control and Cruise control work from the same vacuum source? My truck is stuck on defrost but the cruise works fine? The 4x4 light comes on but not sure how to confirm if it is actually engaging?

 

What year & engine is in the truck?

 

If not used for an extended period, the bend door can stick shut.
This is due to a deteriorating blend door seal.

 

Blend door only controls temp, not where the air goes. If the vents stay on defrost, that is a vacuum issue. The little plastic line going into the cab by the a/c lines likes to break. Cruise gets its vacuum source elsewhere.

 

2001 with the 5.9

 

If 4wd light is on, there's probably a leak at the CAD near the axle. Look at your vac lines carefully. I had an elbow fall apart from dry rot. If all the lines are good, check the check valves on top of the engine. New ones can be had on amazon. I recommend AC Delco/GM brand for longest life.