The knuckles don't go bad. It is mostly just a hunk of metal that the wheel bearing goes into, the shock goes into, and the lower control arm pivots. I think you might be able to warp one. If you ran it hard enough or destroyed a wheel but as for needing replacement? Probably not. Wheel bearings yes, Shocks yes, lower control arms yes.
To get the knuckle off you have to press the drive shaft out of the old one and back into the new. Also it might be cost effective to buy a new knuckle instead of new wheel bearings to install in the existing knuckles, labor is what I'm thinking of there.
