I bought my 4.10 diff from a member who had SZ's guy install them. They make some noise at certain cruising speeds. With a louder exhaust like my 3" B&B, I couldn't really hear the whine, but with a quiet Stillen or Apexi I could hear whine at cruising speeds and it was annoying for me who used the car as a DD.
I have my 4.10 pumpkin out of the car and for sale if anyone can locally pick it up.