Well, as you probably already know, you're gonna have to do some testing and playing around to figure this one out.
First off, check the fluid, a) its simple and b) it needs to be done regardless. No its probably not the source of your mysterious noise. Open the fill hole and simply put your finger in there, the fluid should be up to or right next to the fill hole. If you need to fill it, put 75w-140 in there (though I wouldn't replace it at this current point unless needed to, wait until you're sure you don't need to open up the diff).
While you're at it, go buy some transfer case fluid from your local Jeep dealer. Make sure you get the fluid for the NV247, it will say it on the bottle. (There have been reports of the dealer occasionally giving out the wrong fluid). You'll need two quarts and a 10mm hex socket/allen key. When you go to change it, make sure you loosen and remove the top plug first, don't want to drain the case only to find out you can't refill it. The transfer case has been known to groan when the wrong fluid is put in (is as often the case if the vehicle was ever taken to a common lube up shop), though the sound is not what you describe. This is another case where doing it is easy and relatively cheap and then you know for a fact that the right fluid was put in and have a mileage interval from which to project the next fluid change. (Refer to this: Maintenance Schedule.
Alright, now that we got that out of the way, lets look into the rear. Have you jacked it up and taken the rotor off to see what is going on within there? You say the ABS light is on, do you know why its on? Could be one of the wheel sensors.
What I believe you should do is jack the rear up (both sides), get it on jack stands and then get down to it:
1. With both wheels up, try to spin each. They should spin freely. See if you hear any noise when doing so and can determine further where its coming from here. If you are able to spin both wheels freely, this also rules out the sticky parking brake. If not, adjusting the parking brake may be necessary. Also, just one more time (I know you said you tried), try to see if you have any play in the wheel bearings (do on both sides) by attempting to rock the wheel on the axle.
2. Take the wheel off and begin to take the brakes off of the assembly. I would now again spin the wheels and see if you hear any more noise to determine if it was in the brake assembly or in the axle/bearings. If the noise sounds like its on the outsides of the axles I would bet that its the wheel bearings, if its from the center, you may have larger issues on your hands and need to open up the diff to see whats going on in there.
If you wish to check the straightness of the axle shafts because you believe they may be bent, there is a simple way, to do it at this point. Get something stationary with a straight edge (like a bucket with a piece of 2x4 on top) that you can put up to the axle shaft end (where the lugs are) while someone spins the axle at a fairly constant speed on the other side. Now while the axle is spinning, put your stationary object right up to the edge of the plate where the lugs are and watch to see if the gap between the object and axles shaft plate changes at all. If it goes closer and then further as its spinning, then your axle shaft is bent and needs replacement.
I would bet its the wheel bearings personally, but only because its been an issue with my WJ. I know you don't want to replace the wheel bearing, but its really not a big deal if you have access to a shop press (Some do it without one by using a combination of heating the bearing and freezing the axle shaft, but I had access to a press) and its one of those things that once you do it, at least you know its been done.