  In this paper, we investigate the possibility of explaining both the R_b
excess and the R_c deficit reported by the LEP experiments through Z-Z' mixing
effects. We have constructed a set of models consistent with a restrictive set
of principles: unification of the Standard Model (SM) gauge couplings, vector-
like additional matter, and couplings which are both generation-independent and
leptophobic. These models are anomaly-free, perturbative up to the GUT scale,
and contain realistic mass spectra. Out of this class of models, we find three
explicit realizations which fit the LEP data to a far better extent than the
unmodified SM or MSSM and satisfy all other phenomenological constraints which
we have investigated. One realization, the \eta-model coming from E_6, is
particularly attractive, arising naturally from geometrical compactifications
of heterotic string theory. This conclusion depends crucially on the inclusion
of a U(1) kinetic mixing term, whose value is correctly predicted by
renormalization group running in the E_6 model given one discrete choice of
spectra.
