  Microwave frequency conductivity, Re($\sigma_{xx}$), of high quality
two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B)
is measured with the carrier density ($n_s$) of the 2DHS controlled by a
backgate bias. The high B insulating phase of the 2DHS exhibits a microwave
resonance that remains well-defined, but shifts to higher peak frequency
($f_{pk}$) as $n_s$ is reduced. Over a wide range of $n_s, f_{pk} \propto
n_s^{-1/2}$ is observed for the two samples we measured. The data clearly
indicate that both carrier-carrier interactions and disorder are indispensable
in determining the dynamics of the insulator. The $n_s$ dependence of $f_{pk}$
is consistent with a weakly pinned Wigner crystal in which domain size
increases with $n_s$, due to larger carrier-carrier interaction.
