The F-150 8.8s are 31 spline. Rangers had (presumably still have) a 28 spline version. Bear in mind that Ford axles use a better alloy in their axles than some other mfrs. Ford uses a recipe close to 1050, which has a higher carbon content than the more common 1035 or 1040. A generic 1050 steel, properly heat treated, is about 27 percent stronger than a properly heat treated 1040 grade. Exact steel recipes and heat treat varies, so the actual numbers of each could be a little higher or lower.

Some years back, I sent a bunch of axle shafts to Warn to be destructively tested. Just to know. Included was a 31 spline 8.8 shaft from an Explorer. Warn had a machine that could generate about 6,000 pounds-feet and record the point at which the axle broke. It had no trouble snapping 1040 30-spline Dana shafts (at about 4,500 lbs-ft). It couldn't break the Ford shaft... and the engineer really tried!! On paper, an 8.8, 31 spline shaft has a yield strength of about 5100 lbs-ft, but because it's a fluted design, it's a little stronger than just the steel would indicate.

 

That's good to know, thanks for the info. Now would it be better to put a locker or a posi in the rear end??