98 F150 rear gear ratio
Here's a link to an [link:www.ford-trucks.com/articles/axlechart.php|axlechart] on this site. It's by no means complete for every year, but the codes may not have changed much, if at all.
Anyway, it's reliable information and a good place to start. You'll find the code listed at the bottom on the placard located in the driver side doorsill.