The differential in your car is probably a Ford 8.8. You should have independent rear suspension with half shafts containing CV joints. The half shafts have splines at both ends. The differential side set in place with a flexible C clip. On the outside they bolt in place when the wheel is off.
You may have a bad CV joint. I bought a complete new set of stronger half shafts from The Drive shaft shop.
http://www.driveshaftshop.com They're familiar with broken Superformance half shafts and would be able to produce correct fitment.
I also replaced my Ford 8.8 limited slip innards with a Detroit Truetrac.
http://www.detroitlocker.com/DT.htm It works better.
I got out of the entire job with upgraded parts for about 2000.00