The new diffs should be 100% compatible with the stock diff case (A431). Sorry if this comes across as an "is it plugged in" suggestion, but are you sure you've got the screws lined up with the holes right? More than a few times, I've tried to reassemble my diffs and accidentally had the gear rotated 90 degrees from where it should have been. You can get the screws in, but 2 of the 4 screws will jam into the inner gears, stopping them from turning.