i was leaving my yard earlier(ice strom today) put it in 4x4 worked fine. went to pick up my son at girlfriend and put in 4x4 to get in drive way and it would not work. tried it 5more time on the way home and when i got home and nothing. i hear it clicking but no 4x4. it done this one time before.

 

I assume all the excursion's have the Electric shift system, so when you change the **** does the heater go to the defrost setting. If so you have a vacuum leak and the hubs are not being auto engaged. At the front wheels manually turn the selector from auto to lock, and then see if the 4x4 works

 

Sounds like it could be the shift motor on the Xfer case.

 

I bet its the hubs, like before mentioned, manually engage the front hubs. If not check for vacuum leaks and if you don.t find any the lovely expensive seals behind the front hubs are probably bad.