Truck won't switch back to 2wd on 2005 Chevrolet Silverado 2500 HD

2005 Silverado Duramax that just turned 100K miles. Ran the truck in 4H this morning on way to work due to heavy rains and everything was fine. When I went to switch it back to 2WD, nothing happens. It will go into 4L and back to 4H but not into 2wd. The push button seems a little "mushy" compared to the 4L and 4H buttons. When I turn the key to the "On" position, all three selector lights illuminate for a couple of seconds plus a little red "N" light

Asked by for the 2005 Chevrolet Silverado 2500 HD