what would make the rear end of a 99 Ford Escort sway like driving on ice

Rear end sway bar links. Cost about $15 to 25 for the pair, total DIY project. If the sway bar itself looks gnarly you can replace that too, the bushings that hold it in can get crusty causing it to make noise and move around. ---- https://www.youtube.com/watch?v=YPYeSzYUJAY ---- http://www.ehow.com/how_7633245_install-bar-links-ford-escort.html --- or just Google "replace rear sway bar links" for your make/model/year vehicle.