Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vac is removed the spring engages the IWE.
Lift and support the front of the truck so the front wheels are off
the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.
- If the front drive shaft does not turn :
of the truck ( still jacked up, and supported ) remove the vac lines
from the IWE. The spring should push the gears in, without the vac line
Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
If the vent line is clear, and with the vac lines off the IWE should be engaged ( both front wheels locked ).
they lock, the problem is with the IWE solenoid not shutting off vacuum
to the IWEs. There is a TSB on the 2004-2006 IWE solenoids, a newer
revision has been made, and used from 2007 and newer MYs.
Here is a block diagram of the IWE system
Here is a link to the TSB for the IWE solenoid in PDF format
Start with testing to see if the front drive shaft turns while in 4WD to know which way do go with the repairs, the usual route is the IWE vacuum system ( lines or solenoids ), but I have seen shift motors be a problem also.
If you find a vacuum problem, you can remove and plug the vacuum lines at the IWE, and leave the front hubs locked in all the time, until you can diagnoses where the issue is at.
Just don't try racing anyone or expecting stellar acceleration , pushing a dead axle is going to consume some HP.