1.) First things first. Secure your Mustang so that it is safe to work on. After it is safe and secure, remove the differential cover. While the oil is draining, remove the rear wheels.
2.) Be sure to recycle the used oil. This is what you'll see with the differential cover off. The wire you see is for an oil temperature unit which your Mustang will not have.
3.) We are going to remove the thrust block. The thrust block is located in the center of this picture and is held in place with a special bolt that runs through it.
4.) Use the proper wrench to remove the special bolt that holds the thrust block in. Be careful not to round the edges off the bolt head if you plan to reuse it. It will be very tight!
5.) As the special bolt is removed, you can see just how specialized it is. I suggest that you replace it but you'll have to make that decision for yourself.
6.) With the bolt removed, you can pull out the thrust block. The thrust block keeps the proper distance between the axles.
7.) With the thrust block removed, you can see the gap between the ends of the axles.
8.) Remove the rear calipers. Your Mustang will probably be equipped with the dust shield that will need to be removed also.