  When strong spin-orbit coupling removes orbital degeneracy, it would at the
same time appear to render the Jahn-Teller mechanism ineffective. We discuss
such a situation, the t_2g manifold of iridates, and show that, while the
Jahn-Teller effect does indeed not affect the j_eff=1/2 antiferromagnetically
ordered ground state, it leads to distinctive signatures in the j_eff=3/2
spin-orbit exciton. It allows for a hopping of the spin-orbit exciton between
the nearest neighbor sites without producing defects in the j_eff=1/2
antiferromagnet. This arises because the lattice-driven Jahn-Teller mechanism
only couples to the orbital degree of freedom, but is not sensitive to the
phase of the wave function that defines isospin j_z. This contrasts sharply
with purely electronic propagation, which conserves isospin, and presence of
Jahn-Teller coupling can explain some of the peculiar features of measured
resonant inelastic x-ray scattering spectra of Sr_2IrO_4.
