You would have to be way off on tire size and suspenion to cause this. Your probably coming up on your rpm to slowly. Roll thru the water to the edge of the water box and strike your tires hard. They should almost jump up and start spinning, making sure your in high gear as quick as possible. Your car should track straight then.
One way to do a quick check on your preload is to jack the rear end up and check if the drivers side is coming up first. It should come up about 3/4 of an inch before the passenger side without you in it. Just a quick way to make sure your in the ballpark.
good luck