If you own a 1979-2014 Mustang, then it came equipped with one of the two Ford rear ends – Ford 7.5 rear end or the Ford 8.8 rear end. All 79-85 Mustangs featured the 7.5” rear end. Starting in 1986 Ford introduced the popular 8.8” rear end in the Mustang. From 86-2010 the Ford 8.8” rear end went into all V8 Mustangs (5.0L, 4.6L and 5.4L) and the Ford 7.5” rear end went into all 4 cylinder (2.3L) and V6 Mustangs (3.8L). The 99-01 and the 03-04 Mustang SVT Cobras were equipped with an Independent Rear Suspension (IRS) what used the Ford 8.8” Differential. Starting in 2011, both the V6 (3.7L) and V8 (5.0L Coyote) Mustangs used the Ford 8.8” Rear End.
Ok so you know what Mustang rear axle assembly went into what years – now let’s look at what axles are in your Mustang rear end. All 79-04 Ford Mustang Rear Ends used 28 spline axles except for the 2001 and 03-04 SVT Cobras – they used a 31 spline axle in their IRS rear end. The 05-10 V6 Mustangs with 7.5” rear end used 28 spline rear axles whereas the 05-10 V8 Mustangs with 8.8” rear ends used 31 spline rear axles. For 2011-2014 Mustangs, both the V6 and 5.0L V8 Mustang were equipped with 31 spline axles.