Two things to look for if you can't get your diff to tighten up. First, whatever spacers, diff cone, thrust washer, bel like washer, or whatever you are using under your diff nut, that stack of spacers must be tall enough so that the end of the graphite axle shaft winds up below the outer face of the outermost spacer. If not, the diff nut can contact the end of the graphite shaft befor putting the necessary pressure on the outer diff cone. Secondly, make sure that you have an unflanged bearing in the spur gear and also in the inner part of the diff hub. A flanged bearing in either location will not allow the diff side hub to move inboard far enough so that the diff rings squeeze on the diff balls before the 2 bearings within the diff assembly rub on each other. And of course, it is important that the outermost bearing is a flanged one and that it is nice and free and smooth. A crunchy outer bearing will make the diff feel very poor.