Speedometer

I have a 2001 Ford F150 King Ranch 4x4. It has been sitting for the last 2 days due to the fact it wouldn't move in drive or reverse. It seemed to be stuck in neutral. Anyways we finally got that to work and when I took it out for a drive, I noticed the the Speedometer said I was going 50 and I was barely going 20. And now it seems that the gears are shifting into gear to soon. I have had A LOT of people tell me it could be my transfer case, could they be right? Any help would be greatly appreciated.


Eddie