  A 2.5-dimensional particle-in-cell code is used to investigate the
propagation of a large-amplitude, superluminal, nearly transverse
electromagnetic (TEM) wave in a relativistically streaming electron-positron
plasma with and without a shock. In the freestreaming, unshocked case, the
analytic TEM dispersion relation is verified, and the streaming is shown to
stabilize the wave against parametric instabilities. In the confined, shocked
case, the wave induces strong, coherent particle oscillations, heats the
plasma, and modifies the shock density profile via ponderomotive effects. The
wave decays over $\gtrsim 10^2$ skin depths; the decay length scale depends
primarily on the ratio between the wave frequency and the effective plasma
frequency, and on the wave amplitude. The results are applied to the
termination shock of the Crab pulsar wind, where the decay length-scale (at
least 0.05") might be comparable to the thickness of filamentary, variable
substructure observed in the optical and X-ray wisps and knots.
