The coupled rotational-translational excitations in (KBr)1-x(KCN)x were studied by inelastic neutron scattering for concentrations 0.008≤x≤0.20. We followed the A1g-T2g tunneling transition and the A1g-Eg librational excitation through the transition from the paraelastic to the structural glass state. We found that these two excitations and their coupling to the lattice strains exhibit a very different temperature dependence in the glass state. While the tunneling transition, which triggers reorientations of the CN- ions, shows a drastic reduction of the T2g rotation-translation coupling, the librational excitations conserving the CN alignment are less influenced. These results suggest that for T≤TF the CN system is gradually blocked in clusters, thereby reducing the density of free-ion tunneling states in favor of cluster-reorientation modes effectively decoupled from the lattice. This leads to a metastable low-temperature state with extremely long relaxation times.