FWIW, while I don't offer any kind of warranty on parts I don't typically charge labor to tear into the axle to figure out a problem. I say this because I've only had to do it once or twice out of the hundreds of axles I've set up between work and home.
If you're only going 4.10's you can find stock 4 cyl XJ axles with those gears in them from the factory. They can be hard to find though and they'll be of an unknown mileage/condition. If you have your axle geared and setup properly (by any reputable person/shop) then you're essentially resetting the odometer on your axles to zero. All new gears, bearings, and seals means you should be worry free for the rest of the time you own the rig.
If you're wanting a shop to take it to then I suggest either Northridge4x4 by me or CrawlerTech. I'm going to guess you're looking at around 400$ per axle in labor plus parts. Figure about $1200 out the door.