A statistical mechanical model previously adopted for the analysis of the $\alpha$-relaxation in structural glass formers, is re-derived within a general theoretical framework originally developed for systems approaching the ideal glassy state. The interplay between non-exponentiality and cooperativity is reconsidered in the light of energy landscape concepts. The method is used to estimate the cooperativity in orientationally disordered crystals, either from the analysis of literature data on linear dielectric response, or from the enthalpy relaxation function obtained by temperature modulated calorimetry. Knowledge of the specific heat step due to the freezing of the configurational/conformational modes at the glass transition, is needed in order to properly account for the extent to which the relaxing system deviates from equilibrium during the rearrangement processes. A number of plastic crystals have been analyzed and relatively higher …