help! 1999 f150 4x4 won't engage

When I turn my electronic 4x4 shift to high, the 4x4 is not engaging.
The light indicator says it's on but the wheels do not engage. HELP!

And this is why I don't like "push button 4x4" Any way more than likely the electronic shifter has gone bad. A friend of mine had a 99 Explorer with the same problem. The switch in the dash may be the problem but probably the shifter on the trasfer case. Just a thought good luck