  We study the anisotropic pinning-mode resonances in the rf conductivity
spectra of the stripe phase of 2D electron systems (2DES) around Landau level
filling 9/2, in the presence of an in-plane magnetic field, B_ip. The
polarization along which the resonance is observed switches as B_ip is applied,
consistent with the reorientation of the stripes. The resonance frequency, a
measure of the pinning interaction between the 2DES and disorder, increases
with B_ip. The magnitude of this increase indicates that disorder interaction
is playing an important role in determining the stripe orientation.
