...agree with the poster above ...the stock diff will loosen up after a while, but the ball diff is adjustable for a variety of surfaces and is much smoother.
That said, if the car originally ran straight and now pulls, it may be that the front wheels are not turning freely, thus causing the car to pull to one side. Spin the fronts with your hand and see if they both turn very freely. If one turns slower, then that is your problem and not the diff; you'll need to take the front axle off and clean out the debris wound around it ...this is by far the most common cause of left or right "pulling". Hope you like working really tiny screws
