As a consequence of cation disorder, the compound La3Ni2SbO9 behaves as a new type of magnetic material, a "relaxor ferromagnet", somewhat analogous to a relaxor ferroelectric, with magnetic domains too small to be detected by neutron diffraction forming below 105 K. This formation of domains was ascribed to the presence of local antimony-rich regions wherein the magnetic coupling between Ni2+ cations is disrupted, thus creating a domain wall. A compound with these properties would usually be classified as a cluster glass, but the origin of the behavior in this case, in a crystalline material with a cation distribution that is ordered in one sense but disordered in another, is quite different. We propose a comprehensive study of this new material using muSR to study local order and dynamics, extending the frequency range of fluctuations already probed using ac susceptibility.