x2... 15v is when your alternator is charging the battery (Good connection at the terminal). The 9v is your battery running the jeep with a bad connection and no charge coming from the alternator.
Like said, clean the terminals. You can use sand paper or a wire brush. They also make wire brush terminal cleaners that work pretty good for just a few bucks. They have one side that cleans the terminal and the other side cleans the inside of the connector. Anything to get them all ruffed up, then seat them and crank 'em tight.
Be sure the surface of your battery is clean as well. Even the slightest coating on the top surface can cause a short that will kill your battery faster. You can use an old toothbrush or scrub brush and some baking soda sprinkled on with water. Sprinkle the baking soda, drip some water on it, scrub, rinse w/lots of water. It will look good as new and the baking soda/water mix completely dilutes any acid residue and left over coorosion.