Hubs in and out/ vent to defrost and back again

[BORDERS]

I've read a lot of guys problems with vacuum issues and understand that you check the simple stuff first. I can't find a leak and the problem doesn't always happen. Sometimes the hubs lock and sometimes they are free. Sometimes the air comes out of the vent and sometimes it comes out of the defrost. Does this mean it definitely is the Vacuum Solenoid Control Valve, or could it still be a leak? The truck is an 07 6.0 with 206,000 miles. I just put hubs,seals,and u joints in it. Thanks in advance.

[dannyboy950]

If you can not find a leak then that pretty much leaves the valve as the only thing left.