Could be cracked vacuum lines to the IWE ( Integrated Wheel Ends ) or a faulty IWE solenoid.
The IWEs are held out with vacuum, and when 4WD is selected, the vacuum is shut off, the vent tubes relieves the vac as the spring engages the IWE gears.
I would lean towards faulty vacuum lines for the IWE system ( to the IWEs themselves, from the vacuum box, or the vacuum box itself ).
You can check them by disconnecting the vacuum line from the IWE, and checking with a vac gauge or apply vacuum with a pump.
Start with selecting 4WD ( engine running ) , so you can test to the IWE solenoid from the IWEs, and then work back towards the vacuum reservoir ( located behind the battery box ).
The IWE solenoid could be the problem, there is a TSB where the revision used on the 2004-2006 could ingest water, and introduce it to the IWEs, causing a problem.
Here is the block diagram of the IWE vacuum system:
This is a PDF version of the TSB on the IWE solenoid, so you have it handy :
http://my.voyager.net/~sscully/file_tsb06_08_15.pdf
If you do not have time to get to the testing right away, I suggest pulling the Vacuum lines to the IWEs, and capping them off. This will leave the IWE engaged ( front axle locked, but not beign driven ), do you don't have the engage, disengage problem, which could damage the teeth on the IWE gear. Just watch how fast you try to drive, the truck will not like pushing a dead axle at 95 mph.
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