Could it have had ABS? Weren't all ABS cars 8.8?7.5" 28 spline.
Incorrect my 96 Cougar was ABS with only a 7.5"All ABS cars got the 8.8 diff because the 7.5 housing does not have the mounting provisions for the wheel speed sensors. However which diff you have is irrelevant to the axle. All were the same length and 28 spline count. The axles for the 3.8 were smaller diameter and weaker, so you wouldn’t necessarily want to put 3.8NA axles in a V8 or SC, but given that you have a 3.8, any MN12 or FN10 axle will work.