We investigate picosecond-nanosecond dynamics of the Rho-GTPase Binding Domain (RBD) of plexin-B1, which plays a key role in plexin-mediated cell signaling. Backbone (NT)-N-15 relaxation data of the dimeric RBD are analyzed with the model-free (MF) method, and with the slowly relaxing local structure/molecular dynamics (SRLS-MD) approach. Independent analysis of the MD trajectories, based on the MF paradigm, is also carried out. MF is a widely popular and simple method, SRLS is a general approach, and SRLS-MD is an integrated approach we developed recently. Corresponding parameters from the RED dimer, a previously studied RED monomer mutant, and the previously studied complex of the latter with the GTPase Rac1, are compared. The L-2, L-3, and L-4 loops of the plexin-B1 RED are involved in interactions with other plexin domains, GTPase binding, and RED dimerization, respectively. Peptide groups in the loops of both the monomeric and dimeric RED are found to experience weak and moderately asymmetric local ordering centered approximately at the C-i-1(alpha)-C-i(alpha) axes, and nanosecond backbone motion. Peptide groups in the alpha-helices and the beta-strands of the dimer (the beta-strands of the monomer) experience strong and highly asymmetric local ordering centered approximately at the C-i-1(alpha)-C-i(alpha) axes (N-H bonds). N-H fluctuations occur on the picosecond time scale. An allosteric pathway for GTPase binding, providing new insights into plexin function, is delineated.