Resonant excitonic polariton scattering from ZnSe-like 1LO, 2LO and 3LO phonons in strained ZnCdSe/Zn(S)Se superlattices at room temperature show spatial dispersion effects leading to different Stokes and anti-Stokes frequency shifts. The description in the excitonic polariton formalism by a positive and a negative branch is supported by the strain depencence of the frequency blue shift found for excitations near the resonance of the quantum well excitons. An empirical relation of the negative excitonic mass on the resonance frequency shift is given and discussed in terms of a strain-induced change of the valence band.