you look on the door panel and see what it says for axle code.
then, to answer your next question, you search on this forum for "axle code" and it will then bring you to the post showing which axles/gear ratios match up with which axle codes as shown on your door sticker.
2001 Ranger XLT 4.0L SOHC
Silver Frost w/ Dark Graphite interior
Summit 3" Body Lift
31x10.50 A/T tires
Explorer overhead console
Explorer auto-dim mirror
Aux1 & Aux2 inputs on factory AM/FM/CD/Cassette
Door-dinger silencer button
Seamless retractable roof and passenger-side ejection seat with steering wheel mounted controls and safety interlock