I use the teraflex driver side light mount also. I added a heavy version spring to it to help with trees ect. The springs come in light and heavy. after about 3 months the light version will flop all over when going down the highway. I could see the rack being a problem with longer range talking but for trail to about 2 or 3 miles I doubt it will do much, if tuned for swr. Mounting below the roof line has an effect on "ground plane" of the antenna. SO basically anything that's not on the top of the vehicle will in some degree interrupt your range.
You may want to try two mounts, one at the tail light with a 4' firestik and spring then one on the rack itself with a 3' firestik and spring while using the optional ground wire to ground to the jeep chassis. It would be about 30 extra bucks to get set up for both and try them out. It might be possible with the spring on the 3' you may be able to tie down the antenna to the rack while traveling if it bothers you sticking up all the time.