8 3/4" sure-grip swap and rebuild questions

Backlash is how deep the pinion is sitting into the ring gear. So it most certainly can affect any gear whine.
I may be a dipstick and not know much from only been around for a while but I have found the looser the backlash is the more likely you are to have a noise. And not to forget the clunk when you back off the throttle.
Personally I think it's the brand of Gears that you got there they are famous for making noise. But when you're running open headers you don't really notice it.