The companies had announced last June a plan to trade cable-TV systems in a number of cities in order to increase operating efficiencies and consolidate a number of systems. In exchange for its cable-television systems in North Little Rock and Jacksonville, Ark.; Point Pleasant Beach, N.J., and Louisville, Ky., Los Angeles-based Times Mirror will get Miami-based Storer Communications systems in Phoenix, Mesa and Paradise Valley, Ariz., and in Laguna Beach. Storer also will receive an undisclosed amount of money in the transaction. Following the action by the Justice Department's antitrust division, the transaction still needs approval by local cable franchising authorities in each city.