Many stock axles are too weak to handle rock crawling, large tires, V8s, etc. Coveted axles include the Ford 9", Dana 44, 60 and 70, GM 10, 12 and 14 bolts. The Dana 35 and AMC 20 can be built into stout pieces.
For a cheap upgrade the International Harvester Scout used the Dana 44 front and rear. These can often be purchased for as low as $400.00 a pair.
Bob Stickney writes:
"The stock Ford 9" isn't much stronger than the Dana 35. Currie installs beefier axles and tubes when they build them, plus a host of smaller modifications. Also, the 9" has a low pinion angle that can cause driveline angle problems.
MIT in El Cajon, CA builds good quality custom Dana 44 rear ends. Any custom built axle complete with disc brakes and a locker of some sort will run you around $2,000. Good investment, though."
DynaTrac also has a great reputation for custom axles. A very informative article can be found on their site.
Rooster wrote:
"Let me see if I can explain it. The higher the gear ratio number, the lower the gearing, and the easier it is to turn the axle. Like on a bicycle. Going to a lower gear makes it easy to turn the pedals - the wheels move slower in relation to how fast the pedals are turning, but the legs can power them easily - even up hill. The stronger your legs, the higher gear you can be in and make the pedals turn still.
So, with the 2.5-liter engine, the engine is not as powerful, so the gears are lowered (higher number - 4.11) to allow the engine to be able to turn the wheels efficiently in road and highway conditions. The 4.0-liter engine is stronger and does not need the gears to be as low to get the same effect. For the strength of the 4.0-liter engine, with stock size tires (p215/75), 3.07 gears are fine - and very good on the highway. When the tires increase in size, it gets harder to turn them. So, with the 30" package, the gears get lowered to 3.73 to keep the engine running at about the same efficiency. When people put in 4.11 gears with a 4.0 liter engine, it is either because they went to an even larger tire size and/or want to be able to crawl slower off-road. Like on a bicycle again, in first gear you can keep moving forward very, very slowly, but in a higher gear you just go too fast even pedaling slowly.
Also, as far as being fast off the line, the lower gears [higher numerically] are easier to get turning and get moving quicker - less initial force to get the revolutions going - therefore faster off the line, but not necessarily faster once forward momentum is achieved. If you put 4.11 gears on a Sport with a 4.0 liter engine and p215/75 tires, it would jump off the line fast and accelerate quickly, but would run at much higher RPMs once you were going 75 MPH and higher. A Sport with 3.07 gears would take it over at this point if the distance was long enough to catch up, being able to use the RPMs better at that speed. In a short distance, the Sport with 3.07 gears would not have time to catch up."
OK4WD covers this topic nicely.
Axle ratios can be determined by lifting the end of vehicle, rotating both tires at the same speed and counting driveshaft revolutions. For example, almost five driveshaft revolutions per wheel revolution is 4.88. For increased accuracy it is recommended to rotate the wheels two times and divide the result by two.
You may have different gearing front to rear so be sure to check the other end too.
Most 4WD vehicles when delivered are in actuality only 2WD, yes even your Jeep. This can be seen in very low traction situations where only one wheel per axle receives torque from the engine. In 4WD you may find yourself stuck spinning one front wheel and the opposite back wheel.
This is good for dry street driving where each wheel is free to move independently of one another. This causes much less wear on components when turning.