How to read the tag on a Ford 8.8" Rear
The 3 31 means you have a 3.31 gear in that rear end. The space is for a L, which would make it a limited slip rear end. My rearend has a 3L55 which is a limited slip 3.55, but my friends is just a 3 55.
The trucks have a 31 spline axle, cars use the 28 spline.