An ultrasonic shear wave imaging technique is being developed for estimating viscoelastic properties of hydrogels. A needle placed in the medium is vibrated along its axis to generate harmonic shear waves. Doppler pulses synchronously track shear wave propagation to estimate the local speed. Fitting shear-wave speed estimates to the dispersion relation obtained from two rheological models, we estimate the complex shear modulus, viz., elastic and viscous components. The dispersion equation estimated using the standard solid-body (Zener) model is compared to that from the Kelvin-Voigt model to explore the frequency landscape of hydrogel viscoelasticity within the 50-450 Hz shear wave bandwidth. We found both models give comparable estimates that each agree with independent rheometer measurements obtained at lower strain rates, as might be expected form these highly elastic gels.