I'm trying to help a friend diagnose/fix a problem with the steering on his 1996 Taurus wagon. When the vehicle is not moving, turning the steering wheel is extremely difficult, even moreso than if the power steering had completely failed. With even a slight bit of forward or reverse motion (around 5mph is plenty), steering works perfectly well. I tried revving up the engine while stopped in case it was a matter of rpms to the power steering, but that didn't help either. We jacked up the front wheels and took off the tires one at a time to check things out, and getting either wheel off the ground made the steering work properly. Upon visual inspection, the only thing that looked like it might be out of order is the ball joint on the tie rod for the driver's side, whose boot seemed a bit less regular in shape than the passenger-side one. But I didn't get a definite impression something was wrong with it.
Any idea what the source of the problem could be and how to fix it?
Update: Here are some pics of the ball joints and tie rods: