The effects of microstructural rearrangements during the stretching of semicrystalline polymers and the resultant inhomogeneous strains are modeled by rigid spheres embedded in a polymer network. This results in strain concentrations in the network, which is then caused to yield at realistic overall strains. To simulate the collapse of the original spherulitic morphology, the radii of the spheres decrease at a rate dependent on the shear stress imposed on them by the surrounding network. This results in time-dependent behavior. The resultant large strain viscoelastic model is implemented in a commercial finite element code and used to predict shapes of necking polypropylene sheet specimens at 150 degrees C. Rate dependence of stress and stress relaxation are also predicted, and the model is shown to be generally effective in its predictions of shapes and forces up to large deformations.