Go over bump and rear of car feels like it is swinging side to side? What wrong?

Broken rear coil spring due to rust through. A common problem with the Focus Gen 1 & 2. Replace BOTH rear coils or your Focus will not respond well in an emergency manuver.

My 03 did that felt like rear was rocking side to side when over rough spots. Alignment shop said pass tire front was turned in. 109$ later prob fixed but now in going to cm out rear coils as per pacificoasters reply