My 2001 Sonoma 4X4 4 wheel drive stopped working today while driving in snow. The Sonoma has 20,233 miles and has been maintained very well. While turning into a parking spot I heard a thunk up front and the 4 wheel drive stopped working. I put it in 4 wheel low and it worked for a bit and then only 2 wheel from then on. The 4 wheel drive lights light up as they should. The 4 wheel drive had been working fine and was only used a few times in the past several years. Any ideas as to what may be wrong before I take it to the dealer? Any help would be appreciated.