AC susceptibility and muon spin relaxation data have been collected from a polycrystalline sample of LaSr₂Cr₂SbO₉, a perovskite-like compound wherein the unequal distribution of Cr³⁺ and Sb⁵⁺ cations over two crystallographically-distinct six-coordinate B sites results in the onset of ferrimagnetism at ~150 K. The data are used to elucidate the dynamics of the ferrimagnetic domain walls, thought to lie in Sb-rich regions, and suggest an energy barrier of ~0.13 eV to their motion. The muon data confirm that the ferrimagnetic order is a true bulk phenomenon. The behaviour of this material is briefly compared to that of Ni²⁺- based relaxor ferromagnets.