  For a series of samples with 2D electron systems in dilute Al_x Ga_{1-x}As,
with varying x from 0 to 0.8 %, we survey the pinning mode resonances of Wigner
solids at the low Landau filling termination of the fractional quantum Hall
effect (FQHE) series. For all x studied, the pinning modes are present with
frequencies, f_pk, that are consistent with collective weak pinning. For
x>=0.22% we find f_pk vs B exhibits a rapid increase that is not present for
x=0. We find the observed f_pk is much smaller than values calculated with a
simple Wigner solid model which neglects the effects of the disorder on the
charge distribution of a carrier.
