Need a lot more detail. Do you mean when the automatic transmission shifts or do you have a manual transmission and the problem occurs when you push the clutch pedal? What happens when you just accelerate a little? Was the engine or transmission recently removed? What happens when you apply the brakes at highway speed? Why did you replace the ball joints? As a guess or where they diagnosed as worn? Was the vehicle aligned after that? What happens to the position of the steering wheel? Does it stay straight when the pull occurs, or does it turn?