Thanks for the responses. I think I'm going to buy a 9' extension with coupler and test the SWR. If it works, I'll go with it and report back. Otherwise, I'll ditch the 18' and get a 25'. I don't think my 18' will make it. I'm going out the center plug of my rear door to the Or-Fab carrier with very little slack. I got to the glove box with about a foot of slack.
2018 JL Rubicon Unlimited, 6-speed, White
1985 CJ-7, 4-speed, 258 I-6, 3" Springs, 33" BFG AT, Gunmental Gray