The reorientation process of a probe molecule in a supercooled polymeric liquid crystal has been studied by non linear Electron Spin Resonance. The temperature dependence of the correlation time of the spinning motion of the probe τ∥and the collective viscoelastic relaxation, as given by dynamic-mechanical measurements, have been compared. In amorphous samples the Williams-Landel-Ferry law is observed, which in annealed samples is replaced by a dual Arrhenius law broken at the glass transition. A scaling law τ∥∝ ηξ, η being the polymer viscosity, is found in both amorphous (ξ = 4.7 10−2± 2.8 10−3) and partly crystalline samples (ξ = 5.9 10−2± 6.5 10−3).