When it comes to Diffs, you either build them properly or you don't, and with experience you get better at it. The biggest mistake I find people make is that they don't tighten their thrust down enough and therefore they get the bark which is vibration. If it can vibrate, things aren't torqued enough and some balls will have a bit more pressure on them and thus wear more quickly and unevenly than the rest and it's downhill from there. For my break-in I simply hold one wheel and throttle it lightly for about 30 seconds and then do the other side, then switch sides again and feather it a few times, then gun it for a bit, then do the same on the other side and it's done.

When the diff is built properly it can be felt immediately, and if it's wrong it's quite apparent as well. All I do is make sure my rings are indeed flat and I make sure they're free from ALL debris (even fine fibers can make for a rough diff) and use the proper lubes. My Diffs are ultra-smooth and they last forever that way and I only redo them if I have to tear down my Tranny for some reason. I've only ever had 2 diffs feel gritty and that was due to debris getting in. My current Diff has 6 months on it so it will be getting changed soon.