I know you've got a 28spline. Believe me. lol.
Only Rangers to get 31spline are the 'top dawg' model Fx4's from 02-09.
But to answer your question, go out to your truck and look at your door sticker on the driver door. Under AXLE, what does it say? Then head back on the computer, get on FRF and look at the axle code sticky to decode your rear axle. That'll get you axle ratio and axle type.
You can also take a tape measure to your axle tube...should measure just under 3". Or, take a string and measure the axle housing circumference just outside of the ubolts, then plug it into an equasion and solve for diameter. Should measure just under 3".
'01 XLT disguised as a '03 Fx4 Level II.