We report a passive microrheology experiment to measure the linear viscoelasticity of complex fluids at pressures of up to 200MPa. The apparatus incorporates a sealed steel alloy sample chamber with dual sapphire windows into a diffusing wave spectroscopy (light-scattering) apparatus. We validate this high-pressure microrheology technique with 1-propanol-water mixtures and extend the measurement to hydraulic fracturing fluids containing poly(vinyl alcohol) polymer and borate as a transient, physical cross-linker. In the latter, the viscous modulus is higher than the elastic modulus at frequencies omega > 2 x 10 3 s - 1 at atmospheric pressure. The crossover of the viscous and elastic moduli shifts to lower frequencies as the storage modulus decreases with increasing pressure. The crossover shift indicates a decrease in the cross-link density, which is potentially detrimental for hydraulic fracturing fluid performance at down-hole conditions.