Most springs have incredible life cycle, they usually do NOT wear out within the life of a vehicle several times over.
BUT, for some reason, its common for Leaf Springs to sag. I don't know why, I'm guessing the design is prone to damage, i.e. over arch past elastic mode into deformation, etc.
If it was a coil spring or torsion bar, with no damage and you knew exactly what you were getting (part number/ spring rate/ dimension, etc), I'd say yea, get the junkyard ones.
But Leaf Springs, especially considering how cheap new ones are for an XJ, I have to agree with the above posts; Get new ones. If you have to buy a 2nd Junkyard set because of failure or being bad, you'll probably spend more than a new set that you can have confidence in.