I backed out of the 25 degree garage into 12 degree blowing snow and wind this morning, heard a slight pop (that I thought was ice) and then got the Shift Range Inhibited message on the DIC, and the bar underlining the R disappeared. I had hit the 4x4 button upon backing out to allow for better traction. If I put the truck back into 2wd it worked fine, but 4x4 would give me the same symptoms and the RPMs would increase but the truck would not move. I made my trip in 2wd, and tried 4wd again when I got home. It worked fine then. I am hoping that it was either TC drainback (having not driven the truck in 6 days), or ice on the NSBU switch from driving in the snow a week ago.