Building a smooth diff does require some practice. Finding some good diff lube can really help. If you'll PM me your address, I'll send you some that I have (which I used to sell some years back). The course thread on the axle makes it trial and error to get it set just right but you'll get it. There's lots of little tricks that add up, it's just how much time you want to spend on it....considering the class.
Also, on a Legends car, you want to keep the chassis screws just a touch loose. This builds in flex and helps the car get through the corners.