The live axle rear ends on S-197 Mustangs from the factory have been upgraded from the older 28 spline of previous years. The older 28 spline rear axles would hold up to around 400 rear wheel horse power before reaching their breaking point. The 05-10 Mustang 7.5" rear end used a 28 spline Rear Axle, while the 05-10 Mustang 8.8" rear end used a 31 spline rear axle. Starting in 2011, both the V6 and 5.0L GT Mustangs were built with a 31 spline 8.8" rear from the factory. This was good enough for street cars but once you modified your engine or put on some drag radials it was time to consider an axle upgrade.
The more splines the larger the axle is, increasing its strength considerably. A 31 spline axle for the 8.8 is about 30% stronger than the 28 spline axles. Now, even though the newer 31 spline axles are stronger, they are still mass produced with minimal quality control so they roll off the assembly line at a reduced cost. Many Ford OE axles are made of SAE 1055 or SAE 1541 carbon steel and are hardened. This means only a portion of the depth is hardened while the rest of the axle steel remains unhardened.
If you are going to be pushing more than 500 rear wheel horse power and are planning to do some drag racing you will want to strongly consider a high quality aftermarket axle that has been built for the drag strip. Axles that are built to be subjected to the high torque and demanding stress loads are a must for any serious racer.
When looking at aftermarket axles, you will also want to check that your rear differential will be compatible with your upgrade. Moser, Superior and Ford Racing are all excellent brands to choose from. You can either replace your axles with aftermarket versions that have the same amount of splines, or upgrade to an axle with more splines and a larger diameter for added performance. If you choose to use the same number of splines it is easier install them yourself. If you are choosing to upgrade in size you will need to change the rear end differential.
If you are planning on doing a lot of drag racing then you should look into an aftermarket set of rear axles that have what is called a C-clip eliminator. Your factory axles are held into the rear end differential with C-clips that fit into a groove on the end of the axle. If these factory C-clips or the grooved end of the axle break then the axle can come out of the housing tube resulting in the whole wheel flying away from the car. NHRA rules ban C-clip axles on any Mustang running 10.99 or quicker. The C-clip eliminator consists of a new axle bearing and a retainer plate that bolts to the end of the axle housing keeping the axle from coming out of the end of the housing in the event that it breaks.
So, before the next time you dump the clutch on the starting line at the strip, consider upgrading those axle shafts.