2005 Ford F150 4x4 operation

I had the transmission repaired a year ago and had them inspect the transfer case area too and never checked to see if the 4x4 worked or not after the repairs. I locked it in 4x4 (switch on the dashboard) and I felt the front lock in or a "pull" so to speak but the wheels fail to turn to pull the truck up a slick dirt grade? Hasa a wire been left off or did they screw up my 4x4 to get another free warrenty bill payment?

HELP!!!!