You just have to lower the steering rack, not fully remove it. Take the lower mounting bolts off and let it hang by the tie rods. Just be sure you have the steering wheel locked in position when you're doing this, and don't rotate the input shaft of the rack so it's in the right place when you reconnect the new u-joint.
In order for there to be enough space to get the u-joint to slide free the rack has to be brought down slightly, there isn't enough free movement of the u-joint assy to slide it up and off once the pinch bolts are removed.