Jeeps don't work very well for CB antenna mounting in general due to the lack of metal. A roof-mounted antenna on a plastic hard-top will not perform significantly better than a bumper-mount antenna due to not having a good metal ground plane to gather signals to the antenna. The best CB antenna for a Jeep is a 1/4-wavelength whip antenna, at around 9 feet. Mount one of these on the bumper and watch it flop around. A more realistic option is a good quality Firestik or similar mounted to the tire carrier or tailgate. I'm a big fan of the 3' or 4' Wilson Silver Load FLEX antennas. They have excellent performance, and have an enormous amount of bend in them to keep them from getting damaged. I have one mounted to the roof rack of my Liberty and it has been driven into a parking garage with [amazingly] no damage. It didn't sound pretty, though.