Good bloody luck. I spent better than 6 months watching car-parts, craigslist and checking yards and never actually found a 4.10 HP30 in TX, LA or OK. Finally bought a 3.55 one and regeared it. The 8.25 will probably be a little easier, as there were a lot more 4cyl 2wd than there were 4wd.
So here's what I can tell you:
1) Keep an eye on craigslist, too. A lot of guys part out XJ's themselves there instead of taking them to a yard. Didn't find a 410 HP30 there, but I did find both axles I ended up using and there's more XJ's on craigslist than there are in all of the Dallas self-serve yards.
2) Get on google or yellow pages and find smaller yards outside the big city. XJ's have skyrocketed in popularity in the southern states recently, and they tend to disappear from the big-name yards as fast as they appear. The smaller town yards don't tend to have near as many deals with reman parts dealers and whatnot, and tend to have fewer people watching them. That's my experience at least. Just call 'em before you drive out there and see what they can tell you.
If you decide (like I did) to give up and regear, keep in mind that a convenient axle swap is about the same cost as having a shop do the re-gear. I spent about 600-650 parts & labor on my 4.10 LSD disc ford 8.8. Would have been 300ish if I could weld. If you're not comfortable doing gearing yourself, you're probably looking at 150-200 parts and 400ish labor for each axle. Might as well get a 4.10 8.8 or Rubi rear 44 for around the same price.
Also mind the 3.55 carrier break in the HP30's. If you can't find a 4.10 at least try to find a tow-packaged equipped one so you can get 3.73's and not have to buy the carrier and the gears.
I will be in a yard on Saturday which has a '96 2WD 4.0 with tow package. Will take a look at the differential tag for you.
It'll be a 3.73. Only the 4 cylinder models ever came with a 4.10.
I4 = 4.10
I6 + Auto + tow = 3.73 (also has a tranny cooler that you can install pretty easy. Not a good one, but better than none)
I6 + Auto = 3.55
I6 + Manual = 3.07
I made it to the Tucson yard today. The '99 was a 2.5 with manual gearbox. Down to a shell pretty much but the rear end is still in it & has a tag which is stamped "4.10". The '96 is also a 2WD 4.0 auto box with the factory tow pkg. It has the factory trans. cooler which is damaged. Dana 35 rear end but alas no tags so assume it is the 3.73 ratio. Hope this helps some one!
I read this yesterday and all I could think was good luck, in years of going to the JY here with no shortage of XJs I think I've seen maybe 2 4 cyl XJs. Then I was on Craigslist last night and there was a 99 listed with a 2.5 and manual trans.
Yes, the 4 cyl. are uncommon. Some one grabbed the 2.5 & the manual gearbox since I was there a week ago so somebody needed one. This XJ will likely get crushed soon which is unfortunate 'cause it was extremely clean. The diff. is about all that remains beside the unibody shell.
It'll be a 3.73. Only the 4 cylinder models ever came with a 4.10. I4 = 4.10 I6 + Auto + tow = 3.73 (also has a tranny cooler that you can install pretty easy. Not a good one, but better than none) I6 + Auto = 3.55 I6 + Manual = 3.07
There's a rare percent to find one with 4.56's. But it's possible.