expedition, 4wheel drive

i have a 1998 ford expedition 4x4 with 4.6l engine. the 2wheel drive works, when i put it in awd the 4x4 light blinks but it doesnt engage the front wheels at all, i put it in 4high it blinks but doesnt go into 4wheel drive, same with 4low. i have owned it a year and the 4h and 4L worked when i got it and the light didnt blink. you can feel it go into 4h and 4l but the front wheels dont spin at all? thanks mike