pretty sure there's a stamp/tag/sticker on it someplace that says what ratio is in the housing. pretty sure though the standards are the same for all locales of these things... v6 gasser and diesel autos got 3.73 gears, i4 gassers got 4.10, v6 gasser manuals got 3.55. we had never got the diesel manuals (and you didn't specify if you an auto or manual) but if the transmissions and transfer cases are the same I'd be wiling to bet the diff gearing is the same as well, between your vehicle and the "donor".
otherwise you can try the old tried and true trick... spin the input to the diff and count the number of times you have to turn it to turn the hub 1 rotation.

the difference between the 3 factory values should be easily noticable (3.55, 3.73 and 4.10)
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2007 Jeep Liberty Sport 4x4 3.7L 6spd
- MOABs with Goodyear Wrangler All-Terrain Adventure With Kevlar 245/70R16
- JBA UCA 4.5
- OME 790/948/90009/N32
- Western Suburbanite 7'4" plow
- DetoursUSA Backbone
- HF 12k# winch
[SIZE="1"]WTB: project liberty near N.NJ. stick shift, 4x4, working motor optional,
CHEAP[/SIZE]