if you go over a bump and you keep bouncing and bouncing and bouncing.... basically the strut is there to control your spring... so if you feel like the spring is just going up and down and up and down.... then you have a blown strut...
see if there's oil/grease/grime built up on/around the strut. when I've had blown struts/shocks on other vehicles it had fluid leaking down from blown seals.
Could be that you flat spotted a wheel bearing... is it making any noises? Have you had an alignment done on your car? Look from the front for any major differences. What about your axle? Is there play in it?
if you're on the freeway and the car feels like it's got a flat tire in the rear and goes side to side when bouncing it's got to be a blown strut. I know my struts are wearing out and my shocks in the rear are gone on my car cuz I threw some eibachs over the stock suspension and killed the shocks within 2000 miles lol