See Figures 1 and 2
All Mustangs are equipped with a Vehicle Certification Label attached to the left door post. The axle code (for the original equipment) is located in the space marked AX on the label.
Basically two different rear axles are used in the Mustangs covered by this information: the Integral Carrier, 7.5 inch ring gear and the Integral Carrier, 8.8 inch ring gear. But, each may be available in a variety of configurations and gear ratios.
Additional information is located on the tag attached to the rear axle such as ring gear size, gear ratio and build date. The Drive Axle Application Chart in General Information & Maintenance will provide gear ratios from the code you obtain on the Vehicle Certification Label.