Plasmon hybridization theory (PHT) is an analytical model developed for understanding plasmonic interactions within complex metallic nanostructures, and gives a useful insight for optimizing design parameters. However, this theory is based on the electrostatic limit, which restricts the model to nanostructures much smaller than the free space wavelength of light. Here, we extend the PHT to incorporate retardation of the Coulomb interaction and investigate the effects of retardation on plasmons within metallic structures. We compare these results using other methods, such as Mie scattering theory and the finite integration technique, and observe a good agreement in both electrostatic and retardation regimes. Plasmons within metallic nanospheres and nanotubes are shown to have significant retardation in certain regimes, causing red-shifting to plasmon wavelengths, and we discuss the implications of retardation for plasmonic devices.