Car is jumping right

I have a 96 Grand Prix. Recently I have noticed my car being very unstable at high speeds. When I hit bumps my car dives pretty hard to the right. I was pretty convinced it was something with the front end so I took it in and told them what was going on. They gave me an alignment and said the rest of the front end looked good. Needless to say that wasn't it. Through further driving and closer attention I can feel me rear end kicking to the left when I hit bumps. It feels like the ass end actually slides a few inches but I'm not losing traction. If my ass end is kicking left that would probably explain why I'm diving to the right. The car only has 59,000 miles on it and besides this major problem it is a great car. The mechanics said my strut mounts in the back are bad. I can hear them banging pretty hard when I drive. Could this be the problem? I would like to have some idea what might be wrong before I take it back in. Any help would be greatly appreciated. The problem is much more noticable at high speeds.