Your 2000 XJ has a dual diaphragm booster, and either a 1.0 or 1 1/16 bore MC, according to Napa. The MC for the ZJ is advertized as 1.0, and the booster is also a dual diaphragm. So, other than getting an MC specifically intended for rear discs, I don't see any easy, practical upgrades, without going to larger front calipers & rotors. You could possibly awap in a '99-03 Dodge Durango MC, with the 1 1/8 bore. This would actually reduce pressure to the calipers, causing a higher and harder pedal.
You didn't say what combination valve you're using. The XJ's disc/drum valve would cause your rear discs not to function at full potential. So either gut your XJ valve, or install the ZJ valve.