If it is very intermititent in engaging then i would probably think about replaceing the solenoid and start there.
2012 F-150 XLT 4x4 EcoBeast SuperCrew
2007 F-150 XLT 4x4 5.4 SuperCrew - Leveling Kit - Magnaflow Cat Back - Tonneau cover - Traded
2004 F-150 4x4 S-Cab, 4.6, Auto, Edge Programmer - Traded
Everything You Wanted To Know About IWE's