When you installed the springs did you do the final tighten with the weight of the car on them? If you put heavier springs on you might have changed the pinion angle, you stated it has a bump, bump, bump replace that with bind, bind, bind. It could be a pinion angle problem. If you put it in the air and run it I would suggest jack stands under the axle as opposed to a lift so the weight of the car is where it should be to simulate ride height.