Depending upon how old your wifes XJ is.... There may be a color paper tag wrapped around the axle tube identifying the differential type (D35), gear ratio (3.55) and open or tracloc.
You can also locate this same information in code on a metal tag held on with one of the differential cover bolts or locate the build tag listing vin #, color code, axle & gear ratio codes, etc....
Lifting the rear end up in the air trying to get both tires to turn is not a definitive way to identify the tracloc since yours may or may not be functioning properly.
Last resort is while changing out the differential gear lube, you can look inside the carrier for stacked clutch plates under the side gears for each axle.
CharlesS.