I have been running the Pro One axle and wheels and have no problems getting a fresh rebuild to feel 110%. That said, there's no reason the Xray axle can't be built to do the same. The first thing I would do is replace all the bearings (my first guess is that the thrust bearing is toast) and use flat diff rings by either replacing them or sanding them until the groove is gone. Before you install the new bearings it wouldn't hurt to make sure they are good by adding a little side load while you rotate them. I use a small pair of tweezers for this. I have seen bearings that felt fine with no load that when side loaded a little were gritty. While its possible that the diff balls are flattened, it is less likely in my experience. Make sure you're using the AE 6636 lube too.