I just did this with a D44. Over years of pick and pull salvaging I had grabbed various proportioning valves and compared them. The link below shows the difference in spring length. There are other links out there and the one already included is pretty good, but others show that actually the way the plungers seat may be a little different. To sum up my research:
1: You need the guts out of a rear disk ZJ the ones with drums seem to have a valve that is more like the XJ. (not surprising)
2: I looks to me like the housings are all the same.
3: This is all about limiting hydraulic flow to the rear disks in extreme braking situations.
4: Just pulling the seal does not make much sense to me. I can see that it would work but why not just go get the right part?
To conclude
I had a whole valve of a rear disk ZJ and I had the brake booster and MC out so I just swapped it. I actually think that just swapping the piston and rod would be a whole lot easier IF you have access but your mileage may vary and I can only speculate that the housings are the same. You will need to remove and rethread the end cap under spring tension so be prepared. I love the new setup and it is hard to lock up the rear brakes now but they will before the fronts. The worst part of the conversion for me was dealing with getting new calipers. I ended up with a Wagner caliper that had been mislabled and I will tell you that if you try and bleed brakes when your bleed valve is higher than your flexible hose you will not be happy. I only mention this because if you grab the later Crown Vic brakes you will have to deal with the fact that one is designed to be leading and if you make it trailing the bleed valve will be wrong. This second thread covers this.
http://www.jeepforum.com/forum/f11/rear-discs-438001/index2.html
http://www.naxja.org/forum/showthread.php?t=1074326
Good Luck I appreciate that you are trying to do it right.
nick