Fourwheel drive noise when driving in 2wd noise whinning sound

This is a somewhat common problem with the 04-06 MY 4x4 trucks.

The IWE vac solenoid can ingest water, and introduce it to the IWE, or can develop a problem where it does not keep constant vac on the IWE ends, and the gears hang part in ( grinding ).

You can check the IWE solenoid to make sure it will supply VAC to the IWE ( vac keeps the IWE out, remove vac, and the spring will engage it into the gears ).

You can check the vac lines to the IWE, and the solenoid, but I hate to say the IWE solenoid needs to be replaced.

Here is a link to a PDF version of the TSB with the part number for the updated IWE solenoid that the 07& 08 MY trucks have.
http://my.voyager.net/~sscully/file_tsb06_08_15.pdf

Here is a block diagram of the IWE layout, so you can check the vac lines. Hope it is as easy as a cracked vac line. Also check that the vent tubes are clear, and still up into the engine compartment.

You ether need to replace or rebuild the transfer case and check your front wheel bearings