Electron transport through an InGaAs resonant tunneling structure with Rashba spinorbit interaction and magnetic field parallel to the growth direction was studied theoretically. A nonequilibrium Green's function model was used, wherein interface roughness and longitudinal optical phonon scattering are treated in the self-consistent first Born approximation. The model predicts the main features of the two-dimensional magnetopolaron density of states and the secondary peaks in the I-V curve due to both resonant elastic and inelastic scattering. The I-V curves were studied at magnetic fields around the magnetophonon resonance and the elastic and inelastic contributions identified. At these fields (5 to 7 T), the current spin polarization was found to be dominated by the Zeeman effect and significant even in the presence of scattering events.