Axle Spline Count

[Railroad]

I am shopping for a replacement differential for my 20th Anniv MkIV IRS.
I am seeing 31 and 34 spline numbers on the axle splines for the differential.
Is the 34 number a typo or did Ford change the spline between 2015 and 2020?
Thanks,

[Railroad]

Yukon Gear verified all 2015-2021 are 34 spline.

[Logan]

Just as a note to anyone else looking for this info: the inner and outer stubs are different splines numbers. Inner is 34 spline where it goes into the differential, and outer is 32 spline where it goes into the hub/wheel bearing.

All S550 Mustangs from 2015-2021 are the same spline count (34 to diff, 32 to wheel), including the GT350. There are differences between left and right axles, and thickness of the bar, but that information is irrelevant since we are using aftermarket half-shafts supplied by FFR.

Thus, when shopping for differentials only, it is as easy as picking the gear ratio you want... 3.31, 3.55, 3.73, etc, and note that some are cast iron and others are aluminum (about 14lbs difference between the two). The 3.73 Torsen was OEM for the 15-17 GT Performance Pack, and is the most common way to get a Torsen diff, though it will be an iron case. Sometimes buying the entire IRS cradle is the same price as buying just the diff by itself.