An approach is described for evaluating energies and wave functions for rotationally excited states of symmetric top molecules using diffusion Monte Carlo methods The approach is based on the fact that, for many systems, the rotation/vibration Hamiltonian can be modeled by terms that depend on the vibrational coordinates and powers of the components of the rotational angular momentum vector, J In the case of symmetric top molecules with M = 0, the rotational part of the wave function is given by the tesseral harmonics. We construct rotationally excited states within the diffusion Monte Carlo approach by imposing nodal surfaces that are obtained from the roots of the tesseral harmonics. Results are presented for H3O+ and D3O+ with J <= 10. Where comparisons to previous calculations can be made, the agreement is excellent.