The dynamics of intramolecular vibrational relaxation (IVR) for ArCl2 are examined for a wide range of vibrational and rotational excitation. In order to describe the IVR more efficiently, and characterize it more quantitatively, we propose a refinement of the traditional Bixon-Jortner description in which the active states are prediagonalized to simplify the coupling scheme that must be considered. This allows for an explicit determination of the average density of states and average coupling strength for each initial excitation. We find that the IVR dynamics proceed from the sparse regime for v=11, for which the first open dissociation channel corresponds to the loss of two Cl2 quanta, to the intermediate-dense regime for v=25 which dissociates by the loss of 4 quanta. We find that over this range the increase in the density of states is less important than the increase in the coupling strength. For v = 18 we examine the effect of rotation in considerable detail. Initial states that couple via a manifold of 6000 channels can be considered since the calculation is performed on a parallel computer. The effect of increasing J, the total angular momentum excitation, is found to be less than that of increasing K, the degree of rotation about the van der Waals bond. This means that the main effect is not simply an increase of the available density of states due to Coriolis coupling. Understanding the details of IVR in a relatively simple system like ArCl2 should help us understand the dynamics of more complicated molecules. In particular, the case of ArI2 is discussed.