Snapping/Cracking sound in Rear

Have you removed the rear end cover and inspected the gears, clutches and bearings? You'll need 3 quarts of gear oil, some RTV sealer and a bottle of Ford friction modifier.

Does it sound like metal? It could be a lot of things:

1) Torn torque boxes (remove the rear seat and start looking at the sheet metal for tears)
2) Broken shock
3) Broken Sway bar end link
4) Loose gas tank
5) Loose part inside gas tank
6) Loose bumper support bracket
7) Bad D/S u-joints
8) Tranny mount going bad
9) Something loose in the rear end (mentioned previously)

I would suguest that you don't drive the car anymore and get it in the air so you can look at everything with a fine tooth comb.

Put your rear end up on jack stands, with the help of a (compitent) friend start the car and try to re-create the noise so you can find it. Do Not Forget to chalk the front wheels both front and back. Cuase if your gonna be dumb you gotta be tough.