rocking back and forth
How come my car is rocking back and forth
while on the break? Not too often but
occasionally it's been happing?
You may have a transmission solenoid that is sticking, you can try servicing it and also add 1 can of TransX to the fluid, but if it is a solenoid chances are very high that it will have to be replaced anyway. A good transmission shop can plug into the trans and make the solenoids cycle with thier machine and also can read what the vehicles TCM is doing and let you know what it will take to fix it. Don't wait too long as this movement can burn out the bands and clutches, and then it is going to cost.
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.