8.8" refers to the size of the ring gear. Ford has been using the 8.8 for a LONG time. They have been using it in F150s since the late 80's, Explorers, Expeditions, Rangers. They have been used in solid axle and ifs, irs applications. The front diff on all Raptors is an 8.8" differential. The 9.75 axle was something Ford introduced in the 2004+ F150 as a beefier axle. It has been compared to a Dana 60 axle numerous times for its size and strength. However, unlike a Dana 60 axle, the Ford 9.75 is only a semi-float axle.