Check the rear trailing arms. I had this same prob. "back end swinging out" and it turned out one of the arms had a bend in it. If that's the problem the part is only 65.00 from ford, if it's only one side, and they charged me 33.00 to put it on.

Vinny 2000 T-RED CSVT MODS: 18" REV model 2 ST200 struts/Roush springs CTA intake Pipe/K&N Filter Dtec Heatshield Optimized TB True Duals w/ magnaflow mufflers/resonators