Go back to basics.

This will require the assistance of another person.

Follow the steering link from steering wheel to wheels. Wit engine off rock
the steering wheel from side to side.  Look for play in the universal joint
between the steering column and rack.  Look for play in the steering rack an
steering track control arm bushes.  Repeat with engine (power steering
running) running.

Hint!  Place fingers either side of the joint interface.  Relative movement
should be easily feeled.

If this all check out OK then one needs to check front suspespion bushes and
anti-roll bar bushes. If these all check out OK then the rear end dynamics
could be a problem.

Static tracking measurments will not always reveal sloppy bush and linkage
problems.  One has to stress the linkages and feel/observe the relative
movements in real time.

Nick /////