Low-luminosity gamma-ray bursts associated with Type Ic supernovae provide a valuable window into the dynamics of relativistic jets in complex post-explosion environments. GRB 171205A, the nearest such event observed to date, exhibits multi-phase radiative features that challenge standard external shock interpretations. We present a semi-analytical framework that tracks the propagation of relativistic shocks from optically thick supernova ejecta into an optically thin stellar wind. The method couples shock energetics with radiative diffusion and evolves radiation and gas internal energies separately. It is intended for quasi-spherical shells in the optically thick ejecta that evolve toward transparency, at moderate Lorentz factors. By incorporating radiative diffusion into the fluid dynamical equations and evolving thermal and radiative energy components separately, the model captures the transition from a radiation-dominated to a matter-dominated shock regime. Applying this model to GRB 171205A, we reproduce the evolution of its X-ray afterglow—from the early power-law decline with a quasi-blackbody component, through the intermediate plateau phase, to the late-time decay.