Dallas Morning News: Exxon Mobil Corp. chief executive Rex Tillerson, it seems, has not joined the T. Boone Pickens army. Pickens has been stumping for the past two years for Americans to shift to natural gas as a vehicle fuel, particularly for heavy duty trucks. He says the move would help wean the U.S. off of foreign oil, support domestic natural gas, cut energy costs and reduce pollution. Tillerson said he doubts natural gas would accomplish all of that. And he isn’t just promoting his own …