I'm not sure how testing your brakes for lockup on solid ice could give you any useful information to aid in troubleshooting. It does seem odd, however, that the fronts lock and the rears don't. Drum brakes usually lock easier than discs. But skidding is not the same as stopping so lockup is undesirable at any time, and maybe (however unlikely) you have something going on up front that is making them lock up too easily.
I think I heard somewhere the typical braking distribution on an XJ is around 60F/40R. Probably wouldn't take much effort to stop free wheels off the ground no matter what shape the rear brakes were in, so I'd take another hard look at the rear brakes and see what's wrong. A couple possibilities:
First, cleanliness is next to godliness.
Wheel cylinders. You say you didn't change them. If only one piston in a wheel cylinder is working it could stop a free wheel pretty easy but won't necessarily actuate the self energizing brakes properly.
Brake shoes installed incorrectly. The shoe with the most (longer or thicker or both) lining should face the rear of the vehicle. In operation when moving forward, the leading shoe is pushed into the drum by the wheel cylinder push rod, and the drag on the drum tends to push that shoe into the adjuster/transfer bar, which pushes the trailing shoe up against the anchor pin and out against the drum, and that trailing shoe generates the lion's share of the stopping power. Leading & trailing shoes swapped will reduce braking power a lot.
Incorrect initial adjustment. Adjusting for drag after the drum is installed is imprecise. A parking brake that's been tightened up to compensate for wear can foul you up there, as can a lot of other things. At rest, the shoes should be firmly against the anchor pin up top, and fully engaged in the forked ends of the adjuster screw/transfer bar at the bottom. For initial setup, I like to use a BrakeBlok. It's a plastic pin exactly the same diameter as the anchor pin. Lay the shoes in the drum flat on the bench, insert the adjuster screw assembly, then put the brakeblok where the anchor pin would be at the top of the shoes. Turn the adjuster until the shoes are just free in the drum and it is set. Install everything on the backing plate without disturbing that initial setting you made on the adjuster screw and no further adjustment will be needed. This method is easier/faster/cheaper than a caliper. The BrakeBlok I have came from NAPA about 30 years ago. If you can't find one you could use a deep socket if you have one that is exactly the same diameter as the anchor pin.
Improperly installed hardware. For a quick visual, look at the flat area on the adjuster lever that engages the teeth on the star wheel. It should be perfectly horizontal (that is, exactly parallel with the adjuster screw). If it isn't, something is either out of place or worn out.
Bleeding. Everybody seems to have their own preferred method but there's nothing like a pressure bleeder. If you're like me and most folks you don't have one, so just do the best you can. When you are sure all the air is out do a couple more cycles to be double sure. Not sure how your ABS system will affect what you need to do there, some systems are transparent when inactive, others not and require special procedures.
Once everything is in order inside the wheels work back up the system. Maybe a line pinched tight, but that's a remote possibility. A hose that's visually OK could have a inner lining failure that effectively plugs it under pressure. Hoses are cheap and pressure cycles and flexing will wear them out.
I have never actually seen a failed proportioning valve, but I'm sure it could happen.
Master cylinder. A dead one should activate the pressure differential shuttle that turns on the warning light, but we're talking Jeeps here and the light may well not work. A cylinder that holds under heavy pressure but leaks down under light pressure is failing. A cast iron cylinder is cheap and easy to rebuild with a new cartridge kit and light honing. An aluminum one is best thrown away and replaced with a brand new one.
ABS I dunno. That is technology that wasn't around when I worked as an auto service pro. But any ABS system should fail to a straight through braking system as if it wasn't there. 4 wheel ABS is almost magical, rear only ABS, well, I'm not sure what the point is. But that said if I had it I would want it working properly, and if I didn't have the tools and information to fix it myself I'd take it somewhere that I knew could make it right.
