Definately sound like your intermediate shaft needs to be greased. This is a very common problem on all GM vehicles, the grease breaks down on the shaft splines and creates play- it feels like a knock in the wheel while turning and hitting bumps. GM has a new grease out and your dealer should have it in stock- warranty pays .4 to grease it cash time they will probably charge you an hour. Definately try this before throwing your money at the steering box.