So, your main plates are good, your diff balls are good. However, you haven't changed the thrust race as far as I can tell. If you haven't, then I would 100% recommend replacing it - and I would use this one:
http://www.racing-cars.com/pp/New_Ca...4CX/U1954.html
This is by far the best thrust race that Schumacher have, and it's one that some other manufactures drivers choose to use in their diffs as well
I think this is a different one to the one in the kit, but it definitely fits.
What you could do to try and fault find a little more is swap the assembled outdrive from your good diff, with the assembled outdrive of the bad diff. This will tell you that it's something in that assembly - and you've changed the springs!!