it sounds like the dealer screwed up and did not properly set the PCM when they installed it. As for "burning out the cat" sounds like an attempt to sell a new cat. The cat for the 4.0 is the same cat for the 5.2, and your MPG is not that far out of line. The statement that the PCM replacement was a temp fix bothers me right off the top. did they make sure it was properly grounded? are all the connectors properly seated? are the contacts clean? pull the connectors coat them with DeOxit (available at radio shack but not cheap, google it for other local sources) and plug them back in then put wire ties around the housing and connectors to ensure they stay locked in place and see how that goes. if it doesnt help try a DIFFERENT dealer (dedicated JEEP) tell them that you were told that itsounds like the dealer that replaced the PCM did not set it properly per the haynes repair manual for the 97/98 year JGC and see if the can reset it.