your alternator or ECM charging circuit is bad... take your truck to an alternator shop and have them check it out. . they`ll prolly find the problem. . If you're under warranty still, however. . dealer city. . heh. .
ohya. . get a voltmeter and check the voltage on your batteries while running and not running... if its less than 12v you're not charging, and you shouldnt run the truck long.
-j