A previously developed random matrix/transition state theory (RM/TST) model for the probability distribution of state-specific unimolecular decay rates has been generalized to incorporate total angular momentum conservation and other dynamical symmetries. The model is made into a predictive theory by using a semiclassical method to determine the transmission probabilities of a nonseparable rovibrational Hamiltonian at the transition state. The overall theory gives a good description of the state-specific rates for the D2CO→D2+CO unimolecular decay; in particular, it describes the dependence of the distribution of rates on total angular momentum J. Comparison of the experimental values with results of the RM/TST theory suggests that there is mixing among the rovibrational states.