Disorder scattering and spin-orbit coupling are together responsible for the diffusion and relaxation of spin density in time-reversal invariant systems. We study spin relaxation and diffusion in a two-dimensional electron gas with Rashba spin-orbit coupling and spin-independent disorder, focusing on the role of Rashba spin-orbit coupling in transport. Spin-orbit coupling contributes to spin relaxation, transforming the quantum interference contribution to conductivity from a negative weak localization (WL) correction to a positive weak antilocalization (WAL) correction. The importance of spin channel mixing in transport is largest in the regime where the Bloch state energy uncertainty h/tau and the Rashba spin-orbit splitting Delta(SO) are comparable. We find that as a consequence of this spin channel mixing, the WL-WAL crossover is nonmonotonic in this intermediate regime, which can be related to recent experimental studies of transport at two-dimensional oxide interfaces.