Simple test for a seized front axle u-joint: Jack up the front end off the ground with the steering wheel to one side. Then lock one hub at a time and spin the tire. Any binding indicates a seized u-joint on that side.
That's because the u-joint is more than likely seized on one axis. When you steer in 2wd, the axle shaft pivots only on one axis. So, depending on which axis (seized one or un-seized one) the shaft happens to be pivoting on, you may have good steering one minute and wonky steering the next.What amazes me is that left turns initiate this problem, but not all the time.