The only thing I have ever heard of, is tearing of the frame mounting bosses, or maybe it was the frame mounting bar for the rack. Happened to cars that were tracked with Hoosiers on. There was a TSB on it, invloving welding strengthening gussets on the frame, where the rack attaches.
By 2001, the factory had the welded-gusset-fix incorporated on all new models.
Somehow, I don't think thats your problem, or you would have seen the frame damage by now, right?
I was out looking at mine, and of course, there are visible u-joints under the hood. But it also seems that there may be another u-joint behind the firewall, because it looks like the steering wheel axis is not aligned with the shaft, where it dissapears into a box, just in front of the firewall. The shaft right there looks like it is at one angle, but the steering wheel is at another. One more u-joint in there, in that box just in front of the firewall, would account for the appearant mis-alignment. That presumed u-joint could be slipping.
What about the pinion gear, inside the rack housing, at the end of the shaft? If that rotated on its shaft, you wouldn't see any visible symptoms, except for the steering wheel being off-center. Does the wheel turn, if the steering lock is engaged?
What if you put a vise grip on the shaft somewhere, and try turning the wheel again? By gripping it at different points, you may be able to isolate where the slipping is occuring. Use some wood under the jaws, if you want to protect the shaft finish.