2004 Chevy Silverado Clunking sound in steering column.

Hi,

Occasionally at low speeds (or parked), the wheel is hard to turn, then there's a noise and/or vibration felt. The power steering fluid level is perfect (checked while running and not.) I can feel a clunk as though something is loose in the steering column when hitting bumps. I was told that those models need the "half shaft" greased in the steering column? If so, where and how do I apply the grease? Could this be a result of an ailing power steering pump or something wrong with the steering assembly? Thank you.