Nonlinear step shear dynamics of entangled solutions of an ultrahigh molecular weight polystyrene (PS20M, (M) over bar (w) = 20.06 x 10(6) g/mol) in diethyl phthalate (DEP) are investigated. PS20M/DEP solutions with concentrations 0 spanning the range from marginally entangled to highly entangled liquids are used to quantify the effect of entanglement density on dynamics. Step shear measurements are performed in a setting where errors due to interfacial slip can be determined and minimized. Two characteristic "separability" times lambda(k1) = (16.5 +/- 4.7)tau(Rouse) and lambda(k2) much greater than tau(Rouse) are identified, beyond which nonlinear shear relaxation moduli G(gamma,t) = sigma(gamma,t)/gamma can be factorized into separate time-dependent G(t) and strain-dependent h(gamma) functions. Contrary to expectations from theory, both separability times are stronger functions of polymer concentration (lambda(k1) similar to phi(0.7), lambda(k2) similar to phi(3.2)) than expected for a pure Rouse stretch relaxation route to factorability, lambda(k) infinity tau(Rouse) similar to phi(0). On the other hand, we find that a single shear damping function, h(gamma), close to the universal damping function h(DE-IA) predicted by Doi-Edwards theory accurately describes the strain dependence of step shear material response, irrespective of PS20M/DEP entanglement density.