OK, RFI-EMI-GUY has some good ideas.
Here's what I'll add to his:
The ground strap needs to be as short as possible. The shorter the better.
Probably isn't necessary if the bumper is properly attached to the frame, but it should hurt.
1/4 wave length on CB is about 108 inches, so a 102" whip on it's own is going to be resonate (low SWR) above the CB frequencies.
The 102" whips are designed to have a 6" tall spring between the base of the whip and the antenna mount.
Also, having the whip that close to the jeep body is going to cause some unwanted reflections. While it's not an easy solution to fix, it can throw things off a bit. Don't move it until we tackle everything else.
Check the coaxial cable from end to end. Make sure it's not damaged, pinched, cut, etc.
If you have access to a multimeter or a continuity tester, you can perform the following tests:
1. Disconnect the coax from the back of the CB and the antenna base. Using the meter or continuity tester, check for a connection between the center pin of the cable connector and the outer ring. There should be NO continuity. If there is, you have a short circuit in your cable or one of the connectors. Fix that.
2. With the coaxial cable disconnected as above, check for continuity between the center pin of the cable connector and a known good ground on the vehicle body. There should be NO continuity. If there is, your cable is cut, pinched or otherwise damaged somewhere.
3. With the coaxial cable disconnected as above, Using a long piece of wire, test for continuity between the center pin of the coax connector on the two ends of the cable. There should be continuity between the two center pins. If there isn't, there is a break in the cable somewhere. Could be the cable has been cut, or the connectors are bad.
4. With the coaxial cable disconnected as above, test for continuity between the outer ring of the coaxial cable connector on one end to the far end. There should be continuity. If there isn't, cable has been cut or there is a problem with one (or both) of the connectors.
5. With the cable still disconnected, test for continuity between the 102" whip antenna and vehicle body ground. There should be no continuity. If there is, then there is an issue with your antenna mount.
If all of those check out, then the issue is likely with the antenna length or coupling between the antenna and the jeep body.