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When they go bad, you'll know it. Humming type noise, it will get louder when you turn one way or the other with the weight of the truck shifting. I wouldn't bother replacing them if they're not making noise unless it happens to be apart for something else...you never know what you're going to get. The aftermarket ones are just as bad or worse than the GM ones.
I have not seen any of the good quality ones with a lifetime warranty like jackelope mentioned but he may know something i don't (usually the cheapy ones boost the lifetime warranty).
Nope, you get new wheel studs with a new hub unit, rotor slips on and off.
Jim- the problem is much more common on the 1500 half ton trucks.
196k on my half ton. I've done 3 on each side.