I am going to take a SWAG that this is an issue with the IWE solenoid. There is a TSB for water getting into the vacuum system, and the IWE gears cannot be held out. Still want to check this SWAG.
Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vacuum is removed the spring engages the IWE. The IWE solenoid is what controls vacuum to the system, and is activated either by the switch or the 4x4 lever on the MSoF version.
Block diagram of the Vacuum system
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 :
1.Shut of the truck ( still jacked up, and supported ) remove the vacuum lines from the IWE. The spring should push the gears in, without the vacuum line on.
2.Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
3.If the vent line is clear, and with the vacuum lines off the IWE should be engaged ( both front wheels locked ).
4.If 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. TSB # 06-08-15
Right Click Save as :http://my.voyager.net/~sscully/file_tsb06_08_15.pdf
- If the front drive shaft turns:
1.Check the fuses for the 4WD system ( If ESOF system ) . These are #11, #29, #30 and #111. Use a meter to check the fuses while plugged in, there are pins on the back of the fuse for this. Use a known good ground and check each side of the fuse with the other meter probe, with the meter set to V-DC ( check the scale if a setting that it will read at least 13 V DC ).
2.With the key on, if the ESOF system have someone turn the mode selector from 2WD to 4WD while hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ). Some times the shift motor will get stuck ( lack of use ? ) and the dead blown hammer will free it up.
3.There are 2 relays ( in the small engine compartment fuse / relay panel - ESOF only ) for the 4WD system, one is the CW ( **** Wise ) one is the CCW ( Counter Clock Wise ). These are full size relays, that can be bench tested, or a quick remove and replace test with the high beam relay in the cab fuse panel ( Relays R03 ). 1 at a time, remove the CW & CCW relays and install them in the position for the high beam relay, and test the high beams by pulling back on the MFS ( MultiFunction Switch ). If the high beams work with both relays, the CW & CCW relays are not the issue ).
4.Check the connector on the transfer case for the shift motor, to make sure it is not corroded.
It might not be the TSB issues, it could be a cracked vacuum line causing this anywhere along the vacuum line routing. quick check with a vacuum gauge should show if this is the case.