Struts or shocks...?

I'm having a problem with either the struts or shocks on my car (It's a 96 Chevy Lumina). There is a constant, low rumbling sound coming from the back end everywhere I go, and when I even go very slowly over speed bumps, the back end bounces about 5 times. Also, when I drive over rough pavement or potholes, the back end will lose traction, causing it to shift to the side a little. Is this a problem with the shocks, or the struts?