Selec-Trac is the name given to the NP242 transfercase, it has nothing to do with the rear axles. What you're thinking of is "Trac-Loc", which is what Jeep called their LSD setup found in some of their rear axles. Typically this came on axles that had the towing package, but I'm pretty sure the towing package wasn't a requirement for an XJ to get Trac-Loc.
To the OP, I'm not sure if the axle ID tags had it on there. Easiest way to tell if you have an LSD (Trac-Loc) is to lift the rear axle at the pumpkin, then by hand spin one of the tires. If the other tire spins in the same direction then you've got an LSD axle, if it spins in the opposite direction then you don't have an LSD axle. Shouldn't take more than 5 minutes.
Also you should have a tag with numbers attached to your diff on the drivers side with 2 of the bolts holding the cover in place. You can get those numbers and check them at the link below to also find out further info about the rear axle. 2000XJ's method is dead on.
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