[Bruce Edwards]

I have read all of these post and have a question. Did you check to be sure your rack is firmly bolted to the frame? I know sounds dumb but if the rack is not held tight to the chassis it will cause this same kind of problem. Just because the bolts are tight does not mean the rack is tight. If the bolts are to long you may be running out of thread before you get the correct crush on the rack bushings.
One other thing to check, with out seeing your set up I am shooting a bit in the dark if you are running more than two u-joints do you have any "play" in the shaft? This will give a feel like a loose rack at the wheel and making the rack tighter will mask the real problem. Have someone turn the wheel and watch the steering shaft to be sure it stays true and does not bind. If the shaft is moving around with the steering wheel it will beat you to death in the car when the tires try to find the groove at speed.
Had an old car and every time you would get to one speed the whole front end would start to shake so bad you would think it was going to fall off. A quick shake of the wheel right to left and it would stop. Turned out to be a bad joint in the steering and a bad front spindle. The inner wheel bearing had worn a groove in the bottom of the spindle and at speed when the tire started to grow a bit it would start to bounce on the inside edge and that would start the shake. By changing the load on the front end I could stop the shake till I dropped below that speed at which the tire changed shape.
Boy that was long winded. Sorry about that.