2002 Ford F-150 Malfunctioning 4-wheel drive

I am having difficulty with the push button 4-wheel drive. The truck will not go into 4-hi or 4-lo. When I turn the 4WD button into 4-hi the light indicates the truck is in 4WD, however, the front wheels will not engage. There is no sound at all when switching between 2-hi & 4-hi. When I put the truck in nuetral and shift to 4-lo, I hear the "klunking" sound and can feel the transmission engage, but the front wheels still won't pull (or turn). The lights for 4-lo indicate the truck is in 4-lo.