We demonstrate acousto-optic modulation of a fiber Bragg grating in a birefringent suspended core fiber up to frequencies of 5.2 MHz for the first time. At acoustic resonances lower than 820 kHz, the reflectivity of the orthogonal polarization modes is electrically switched to a superposed reflection band with a maximum modulation depth of 51% at 10 V. The wavelength peak of the polarization modes is dynamically shifted by tuning the driven electrical signal from 1.92 to 5.2 MHz. A novel side-coupled based acousto-optic device is demonstrated, indicating new possibilities for ultracompact, fast and efficient all-fiber integrated devices.