A simple "effective media" approach is used to calculate the constitutive equilibrium field Be(H), and hence also the equilibrium magnetization Me(H), relations for a polycrystalline, anisotropic Type II superconductor with random grain orientation. Mutually-consistent scaling of experimental M versus H isotherms to the calculated Me(H) relation, for an as-prepared polycrystalline MgB2 specimen, allows for the determination of a self-consistent set of values for the anisotropic G-L parameters and for the critical fields Hc1(T), Hc(T), and Hc2(T) for the material. The calculated Be(H) relation, together with explicit critical current density, Jc(B), trial functions, allows for the determination of flux density profiles [B(r)]H and also the nonequilibrium magnetization M(H) behavior, which is compared with the experimental M versus H isotherms. Optimum fits are obtained with a Kramer-like relation of the form: Jc(B,T)∝Hc1n(T)(1-B/B0)2B-12/, where B0(T)≈μ0Hirr(T) is the irreversibility field, and n=0.75 and 2.25 for T below and above 28 K, respectively. The general form of this relation suggests that Jc in polycrystalline MgB2 is determined by vortex pinning at grain boundaries.