rocking back and forth

Or do you mean it rocks while you're on the brake but still rolling along? If that's what you're referring to it is likely a tire starting to separate. If so, your front tires need to be replaced immediately. In the short term (I mean for a few days, in town driving and being careful) you could swap front tires to the rear and put the rear ones on the front. A bad tire is much safer on the back of the car than on the front, but again I stress, this is only a temporary measure till you can get bad tires replaced.