I think you can get away by removing some of the material from the diffcup. Just use some emery paper on a hard and flat surface (glass coffee table works great) and and sand the end of the diffcup in circular motion. This should remove enough material. Since the shims are so thin anyway you will only have to remove very little. Just do your best to keep it flat. This will also move the crown gear in the direction you need it to go (towards the pinion).