Nickel substitution at the cobalt site in Na_0.75CoO₂ induces an upturn in the resistivity on lowering the temperature, with the metal-to-insulator transition temperature (T_MIT) increasing with the Ni content. Low temperature far infrared measurements on polycrystalline samples of Na_0.75CoO₂ and Na_0.75Co_0.95Ni_0.05O₂, the latter having T_MIT ∼ 175 K, have been carried out. Dramatic changes in the Na mode frequencies, and relative intensities of the out-of-plane modes corresponding to the two Na sites are observed, coincident with the MIT in Na_0.75Co_0.95Ni_0.05O₂. It is argued that these changes are associated with a charge ordering of the CoO₂ layer, associated with the metal-insulator transition.