[en] The neoclassical moment approach of Hirshman and Sigmar [Nucl. Fus. 21, 1079 (1981)] has been used for the first time on the Joint European Torus [J.Wesson, Tokamaks, 2nd edition (Oxford Science Publication, Oxford, 1997), p.581] to deduce the toroidal rotation frequency of the main ion species from that measured for the impurity ions. A significant differential rotation between these two species is found where the second radial derivative of the ion temperature is large. The position of radially localized magneto-hydrodynamic instabilities is then deduced from the Doppler shift in the frequency and verified using the cross-correlation between the magnetic and the electron cyclotron emission measurements. Accurate agreement between these estimates of the mode location is only found when using the toroidal rotation frequency of the main ion species. This result fully verifies the neoclassical approach and can be used directly as a diagnostic tool to localize magneto-hydrodynamic instabilities. (author)