Do you have automatic hubs? If so they could require a repack. In Virginia winters we used to (and still do on occasion, but not as often now that we know the required maint.) have a problem with the hubs becoming "partially engages" - enough to make a horrible sound, but not enough to actually engage. This would only happen when in 2wd. I'd suggest that you take the hub cover off, then the spring loaded part of the hub. Clean them out well, and replace. See if that fixes it.
-Tad
-Tad