The units were sold through Shelby in 66 as a option but they were VERY unpopular. Also sold through truck shops and part houses. They ended up in allot of 2 ton trucks
Hard to find today, require that you make sheet metal alterations in your cars body.
They came in two versions - one that basically replaced your pinion gear and retainer in a Ford 9" (requires you to shorten your driveline) or bolted to the floor pan requiring that you connect two drive lines (one fore and one aft) in line
Have one in my Fiberfab ET Mustang and have seen a few in 66 GT350's