Static and dynamic light-scattering experiments have been performed on isotropic dispersions of sterically stabilized polydisperse boehmite rods, with an average length to width ratio of 6.4. Measurements were made up to concentrations close to the isotropic nematic phase separation. At high concentrations, the scattered intensity autocorrelation function of the rods is found to separate in two modes that are attributed to collective and exchange diffusion. Corresponding mode amplitudes are in fair agreement with results of the polydisperse scaled particle theory. Whereas the collective mode decay time is hardly concentration dependent, the exchange mode decay time decreases exponentially with concentration.