Picked mine up from a guy in PA in June for $250. Out of an '87 XJ, about a 100,000 miles on the axle, one bent backing plate from carelessness, 3.54 gears w/ LSD, all the brake parts were completely shot.
I would say for the XJ D44 that you described, Kyle, I'd say that $500-$600 would be a good price because of the fact that the shock mounts have been raised already, it has an aftermarket cover, and it has a full disc swap already completed, from what I understand. Adding to the base cost of the axle, $500-$600 for everything is pretty fair IMO.
It looked like this when I brought it home:
Bent backing plate: