Not sure if I can be helped on this but I have a 98 F150 that I just bought and when I shifted the floor shift to 4 wheel drive I could feel the gearing drop but my front tires would not spin. Also I noticed the four wheel drive light on the dash never lit up. I then looked under neath the truck and I noticed that the plug just below the 4 wheel linkage that goes into the transfer case was unplugged. I plugged it back in and now The four wheel drive light comes on but I have not checked to see if all four tires will spin. However now that I've plugged this in I can't seem to get it back into two wheel drive. If I play with the shifter I can get the 4 wheel drive light on the dash to turn on and off. I tried adjusting the linkage and I still can't seem to get it to pop out of four wheel drive. Even If I do get it out, once I start driving and hit bumps then I hear it grinding trying to get into 4 wheel drive.
If anyone has an idea of what could cause this please feel free to email me at email@example.com. Also when I lay under the truck and have my girlfriend shift from 2 wheel to 4 weel I can hear the transfer case click into gear.