  We interpret the linear polarization structures observed in extragalactic
radio sources, even those oriented at oblique angles to the jet flows, to be
due to oblique, relativistic shock fronts in the emitting regions. Many sources
exhibit indications of such oblique structures, and the goal of this
investigation is to test this hypothesis quantitatively. A selected group of
ten highly-variable extragalactic sources were observed with the VLBA at 15 and
43 GHz, on nine epochs spanning a 30-month period; five of these objects were
also observed at 8.0 and 22 GHz. The integrated total flux densities and linear
polarizations of the selected objects were also observed several times a month
at 4.8, 8.0 and 14.5 GHz with the University of Michigan 26-meter telescope.
All objects exhibited variability with several exhibiting more than one
independent outburst during the period. We show the evolution of the polarized
components with time and discuss the relativistic shock parameters required to
match the observed polarization structures. Even cases where the magnetic field
is apparently oriented along the jet flow can be fit by oblique shock models
when relativistic aberration effects are included.
