The CJ's didn't come from the factory twin sticked. That's something you have to modify to the transfer case for. My guess is your XJ just took a better line or flexed a little more, keeping tires on the ground the therefore keeping traction. Tires up in the air with open diff's means you aren't moving forward any more. Take the front sway bar off and air the tires down, with a good line it should have no trouble going anywhere your XJ does.
88 YJ with a 4.6l stroker, AX-15, NP231, 2.5" lift, 4.10's and 33's
2000 Red XJ w/ 350k+, AW4, NP231, Locked Rear, HD Crown Leafs, trimmed to fit 33's, soon to be a donor
2000 Silver XJ, Drivetrain from the Red XJ, Stiffened, IRO leafs, RE coils, custom 3 link, WJ knuckles, 35's <=== In the works