Like @TJ Starting says you don't need to worry about ground on a GMRS antenna like this without a ground plane. If you did have a ground plane like the one @NOTNSUV has all you have to do is connect the NMO ground to the metal ground plane. You don't need to ground to the Jeep. If you want to spend effort to improve your setup grounding the antenna to the Jeep is not the answer, getting a functional ground plane like what @NOTNSUV has is the right way to do it.