2001 Ford F250 Rear Axle Size

if you have the vehicle lifted there is usually a tag or sticker on the diff. If not you can rotate the drive shaft compared to tire revolutions.

on the door panel where wheel and tire info is sometimes it is marked there.

or in the glove bow there is a code sheet, has your paint code etc. near the end of all those numbers it will usually have your ratio.

The easiest way would be call your local ford dealer with your VIN, they should be able to tell you over the phone.

Try entering VIN online it might give you your line sheet, likely have to sign up for something. Try FoMoCo.com

The web page is a complete shot in the dark on my part...