I'm with Kastein on this one. Find one in a pull a part for much less. Sounds like the $130 range is going rate in MA. In Tennessee, I pulled mine for $90.
That one you pictured still needs to be converted for an XJ....spring over, re-run brake lines, and new shock mounts. Also, the one in the photo doesn't have the bracket for the drive shaft (which Kastein alluded to). If you pull one from the PAP you can snag the driveshaft bracket also.
So, you get the $130 axle from the PAP, then spend another $100 or so for someone to set it up (if you aren't able), you are still well under the $500 that you were quoted.
Remember, if the one you pull isn't geared the same as your front, you will need to replace it as well. The Explorer axles are generally 3.73 or 4.10. Tag will tell you. If it shows 3L73 or 4L10 that is the gear ratio with the L indicating Limited Slip.