4x4 will not engage

[05 6.0]

Hey guys, my 4x4 wont engage unless I lock the hubs, then it works fine. Why is this?

[dnf12908]

Possible moisture in your vacuum lines, or if it's cold enough, iced up. I had this issue once.

[twoicebergs]

I believe he is in Missouri. Do a check of your vacuum hoses and follow it up to the collector. It usually would crack near the axle hubs. If you don't find it visually, do a vacuum check of your hubs and of the system.

http://www.powerstroke.org/forum/att...works-esof.pdf