Truck doesn't go into 4 WD. Can turn switch on dash and it will read 4 WD High but it just clicks if you are trying to go but doesn't engage. 4WD was used maybe 15 times in 6 yrs, and never for more then maybe 5 miles total if that helps. It is a family vehicle.
Front wheels don't engage into 4 wheel dr. on 2006 Lincoln Mark LT
the IWE solenoid is a very common culprit for this. IWE = individual wheel end. it's an electrical solenoid that controls vacuum to the front wheels for engaging he front axle. i would have it tested to verify, easy and low cost fix.