Nonequilibrium themodynamics is used in a novel way to fundamentally investigate colloidal back-diffusion during crossflow membrane filtration. A generalized chemical potential is assumed as a superposition of equilibrium and irreversible contributions, which are proven to originate from Brownian and shear-induced diffusion, respectively. As a result, an effective drag force is derived for a particle that is simultaneously undergoing both Brownian and shear-induced diffusion in a sheared concentrated suspension.