You can also pull the differential cover on most axles and see what is going on inside. Lots of rust may mean that the axle is junk…or at least the bearings, gears, and differential are junk. If the hubs are all full of rust and water, it can get expensive replacing parts that otherwise would be reusable. Here the original differential tag indicates 3.73 gears. See the numbers 41 and 11 stamped on the edge of the ring gear? That means the pinion has 11 teeth and the ring gear has 41. Divide 41 by 11 and you get the ratio 3.727272 so these gears match the factory tag. Also, just below the side gear on the bottom of the image, you can see a clutch pack indicating that this TJ Dana 44 has a limited-slip differential.