it sounds like the transfer case is trying to engage while driving without the switch to 4x4. Grinding noise then a clunk

How long has it been since you changed the transfer case fluid? Ford normally recommends doing it every 100,000 miles.

Transfer Case Fluid. Use 2 qts. Of Mobil 1 ATF. Tighten 10mm hex drain and fill plugs to 20 ft.-lbs.