I would check the steering rack mounting bolts as well. I've heard of them backing out before, and that could cause the symptoms you're describing.

A u-joint went bad in the steering of a BMW 320is that I had years ago, and ti would steer fine for the most part but would bind at one particular spot in the steering wheel's rotation. Complete u-joint failure should result in the complete loss of steering. So, I doubt the steering u-joint has gone bad...check the rack bolts.