You can use the search feature to see other posts about using these axles in an XJ, there is a lot of info regarding it. The D44/9" is a great axle combo for an XJ, both are stout and there is tons of gears/locker combinations available for both. The 9" shouldn't be a problem, the biggest complaint I hear is the low/offset pinion. With the length of an XJ's driveline and especially if you go with an SYE in your t-case and rotate it up, should be good to go. The D44 unfortunately has radius arms, as you're aware. From what I know, it's a pain in the ass to use these housings on a standard XJ coil-bucket set-up, as the raduius arm wedges are part of the casting. There was a guy recently here who made radius long-arms for an XJ with this axle, using the factory wedges and locating a coil bucket, search and you'll find his thread.
As far as cost, some of it is static, some of it depends on how much of the work you can do yourself. Gearsets are around $200 each, set-up kits about $100 each, so there's about $600 just in parts. Lockers range from about $250 for a lunchbox to $800 for an Ox selectable, tried and true Detroits are $5-600 each. Adapting the 9" for the rear leafs is fairly simple, adapting the D44 for the front more complex, as I stated above. Labor for setting up gears seems to run between $200-$400 per axle, depending on where you go and who you know.
Good luck researching, if it were me and I came across these axles so cheaply, I'd probably go for it.
