  We show that leptophobic $Z'$ gauge bosons occur naturally in flipped SU(5)
and may shift $R_b$ in an interesting way without upsetting the good values of
$\Gamma_{had}$ and $R_c$. Within a string-derived version of the model, we
study three possible scenarios and the constraints imposed on model building
that would allow the new symmetry to remain unbroken down to low energies. Such
$Z'$ gauge boson has generation non-universal couplings to quarks that violate
parity maximally in the up-quark sector, and may contribute significantly to
spin asymmetries in polarized $pp$ scattering experiments now being prepared
for RHIC.
