I've read that you have to look at the tag on the axle. Sometimes they say it's stamped on a flat spot to the right of the pumpkin as it appears from the rear end of the Jeep. Please correct me if I'm wrong - If you can't find a tag or scrape away enough mud/rust etc., you can try jacking up one rear wheel and turning it while counting the turns of the wheel until you see one full turn of the input shaft on the rear end.
If it takes almost 4 full turns to make one on the input shaft, you probably have 3:73's. It it takes a little more than 4 turns, it's probably a 4:10. Remember to put your transfer case in N so the drive shaft will turn without a bind. If you have a locker, you will have to jack up both wheels and turn one (correct me if I'm wrong).
2004 Rubicon, 6" lift, 35s, winch, hard top, full doors, bumpers, tire carrier and rock sliders.