FMS part #

You're right, that was 5 lug, sorry.

The only 4 lug 31 spline 8.8 axles Ford Racing makes that I can find for 79-93 mustang is part # M-4235-B, but they are semi-floating.
 
M-4235-B is the correct part number. Description is as follows from my parts book.

79-93 Mustang
Heavy Duty 31 Spline Axle Shaft (29.61" long) for applications using 31T differentials
4 Lug, 4.25" Bolt Circle