The most popular rear end out there is the Ford 9 inch. Because it found its way into so many vehicles, there are a lot available. The aftermarket has added thousands of parts to upgrade the 9″ to handle even more power. Aftermarket 9″ Ford rear axles are offered in 31, 33 and 35 spline. Heavy duty drag racing spools and limited slip 9 inch differentials are also plentiful.