Axle codes are the exact same.....door sticker are the exact same. The only way to confirm it to be 31spline is seeing the truck it came out of and/or taking a tape measure to the axle housing to measure circumference. Which, will be significan't bigger than your existing axle. That's actually how I confirmed my axle came out of the truck I said I wanted it out of at the junkyard. Got me a tape, measured my existing axle(8.8 28 spl) then measured the axle on the pallet(8.8 31spl fx4).
'01 XLT disguised as a '03 Fx4 Level II.