Hey guys, I'm having an issue with my front hubs in my 2008 F350 6.4 Diesel Harley... To engage 4x4 inside, it's just the **** that i turn, not a floor shifter.

The transfer case engages, but the hubs won't lock. I have to get out of the truck, take off the wheel caps and manually turn the hubs to "Lock", if I keep them on "Auto" nothing happens, only RWD works...

I think I read somewhere that these "Auto" hubs are vacuum actuated. Could the hubs be the issue and not holding vacuum? Or is there some vacuum pump that could be broken? The truck has 70k miles on it, so it's out of the powertrain warranty where would I start diagnosing this problem?