I just regeared my XJ from 4.10s to 5.38s. I'm running 36" tires and I can tell you that even with the 6cyl, 4.10s and 36s sucked big time. I never got into OD on the highway and anytime my speed dropped below 80KM/h the tranny downshifted to 2nd.
So even if you bought a Rubicon, which is the only way to get a 6cyl engine and 4.10s, you will still need to regear.