Freeplay in steering.

My '92 Miata has about 3/8" to 1/2" freeplay in the steering wheel at all
positions of rotation (measured with the car at rest).
My service mechanic says that he can find no loose parts and no play in the
steering linkages. He believes that the play is in either the rack and
pinion or in the lower universal joint of the steering shaft.
The Mazda dealer says that the lower steering shaft, which includes the
universal joint is no longer manufactured.
Does anyone have experience with this problem?
Is there a wear problem with the universal joint or steering rack?