Because the D30 is limited to a 4.56 and no lower?
It used to be that the 8.25" was the issue. The deepest that you could go was 4.56s.... now it's 4.88s!!!! It wasn't too long ago, neither.
I am running 4.56s because at the time it was the deepest I could get.. less than a year later, my neighbor buys 4.88s..... MF'r!!!!! That pinion gear is soooooooooooooooooooooo tiny for the 4.88s.