Pick ups always have issue with a lack of weight above or behind the rear axle. By putting weight towards the rear of the bed it puts more leverage agains the springs and allows them to work more. Thats a lot of why high speed offroad trucks mount the spre tires so far to the rear despite having no need to save bed space (especially since the bed is pretty much non-existent). The XJ has its fuel tank behind the rear axle, that alone makes a big differance in weight bias on the suspension compared to an MJ's. We also obviously have a few hundred pounds worth of hatch, glass, and roof back there the MJ doesn't.
His springs are also super stiff, I was behind him on the way out and his rear tires were bouncing up in the air wih every rock. Almost zero rear suspension movement. I actually noticed it that morning when he ran down to the Diamond Shamrock for fuel, the way it took the bumps on pavement was like my old Rabbit on race spec coilovers.
At a minimum and just to experiment I'd go buy some sand bags and toss 2-300 lbs worth between the spare tire and the tail gate. See if that helps the on road ride at all. You also need to ditch those Rancho's and get yourself a good set of 5100's or 5150's. Rancho shocks are typically stupid stiff.
If all else fails I think going to a SOA w/ those stock MJ springs will work nicely.