When you took off the (+) you jostled the line from the alternator. The truck isn't getting consistent voltage. This happened to me in a Honda I used to have. Start the truck, disconnect the (+) terminal, and put a voltmeter on the line coming from the alternator. If the truck dies, your alternator is toast. If you get a constant reading I'm wrong. If it's jumpy you either have corrosion in your line, on the alternator stud, or your alternator is almost toast. Good luck!