The modulation phase-shift method is a standard technique for measuring the group delay and dispersion of fiber-optic devices. The wavelength resolution is typically determined by the radio-frequency modulation frequency. Decreasing the modulation frequency improves wavelength resolution at the cost of decreased sensitivity in the measurement of the time delay. We present a technique that simultaneously yields both high wavelength resolution and high time delay sensitivity by combining two measurements made at different modulation frequencies. High-resolution experimental results are shown for a fiber Bragg grating device.