Shakes when hit pot holes

I have a 2004 Kia Spectra I purchased 2-3 months ago. It drives good on smooth surfaces, no issues. However when I hit uneven surface or pot hole the back end seems to shift from side to side for about 1-2 seconds then recovers. I looked under it and didn't see anything broken. One tire looked worn so replaced it. Was told it needed new struts, so replaced all 4 struts and mounts, but it still does this same shaking when you hit a bump or uneven surface. I jacked it up and tried to wiggle the wheels, but nothing loose. Didn't see any brake dragging. It doesn it even if you don't have brakes on. Any thoughts?