  We study electromagnetically induced transparency (EIT) in the
5s$\rightarrow$5p$\rightarrow$46s ladder system of a cold $^{87}$Rb gas. We
show that the resonant microwave coupling between the 46s and 45p states leads
to an Autler-Townes splitting of the EIT resonance. This splitting can be
employed to vary the group index by $\pm 10^5$ allowing independent control of
the propagation of dark state polaritons. We also demonstrate that microwave
dressing leads to enhanced interaction effects. In particular, we present
evidence for a $1/R^3$ energy shift between Rydberg states resonantly coupled
by the microwave field and the ensuing breakdown of the pair-wise interaction
approximation.
