Rear axle bearing?

I have been told by a mechanic my driver side rear wheel bearing needs replaced. Its a Silverado with drums in rear. I've driven on this thing for a couple months changed all the bearings in the front. To find its the rear bearing. When I change this bearing should I expect to have a worn axle shaft too? Should I preemptively purchase one incase. Or is it uncommon for them to be worn. Thanks