Well, I set my pinion angle with the shims that was given to me in my flip. I don't know what the guys are doing who fuck theirs up, but my vibration decreased the lower I went. When I was stock that thing hated 90mph and felt like two fat bitches doing the dougie!
I don't think it has ever rubbed at 8" but I run air so I'd guess I am at 7". Even so, my truck has been driven many times with very little to no air