Not shifting into 4wd LOW

[jac]

Ok 2007 Expedition 78000 miles 5.4 XLT. Using 4 wheel drive yesturday in snow, and noticed not shifting into 4x4 low. The 4x4 auto and 4x4 high work, so we are clear I did the proper (nutr,foot on brake) still nothing. I can hear what soundsl like clicking in dash, and display has "in progress" but after several seconds the "in Progress" goes away and still in 4x4 high. Duing this process if I shift into gear display reads "shift to nuetral" Any help would be great. jim