Jeep Spring Over Axle Lift

According to Rocky-Road.com the SPOA, or Spring Over Axle lift, is by far the most popular with the rockcrawling crowd. This is because articulation is maximized by additional leverage placed on the springs in moving them over the axle. Articulation is what we call up and down wheel travel. Why is it important? Because tires on the ground mean traction. Also, its hard to tip a vehicle over that always has its tires stuck to the ground. With the SPOA, you'll get more side to side deflection of the same spring than it if is in its stock spring under axle position due to this leverage. The Spring Over lifts clobber articulation claims of the latest fad... the shackle reversal.

The folks over at Dehesa4x4 have a lot of good information about doing a SPOA conversion on your Jeep from experiences they had with a YJ, here's a snippet followed by the link:

Driveline Angles - The driveline angles are very critical part of setting up an SOA. On a YJ, the front is not as crucial since the YJ driveshaft is long enough, the u-joints do not have to operate at severe angles. But the rear driveshaft on a YJ is extremely short. Close attention must be given to this area to prevent vibrations and u-joint failures.

On the front perches, you'll simply want them welded parallel to the stock perches underneath. Two reasons, one the front is a standard driveshaft so the transfer case output shaft and the front axle pinion shaft should be parallel. The second reason is, changing the angle of the front perches will affect your caster. Caster is what causes your wheel to return to center after a turn and track in a straight direction. I've heard Rubicon Express recommends changing the caster for their SOA kits. I'd contact them for information. All the SOAs we've ever done kept the caster the same and we've never had a problem in this area.

On the rear perches, it will depend on your driveline setup. With a standard driveshaft, like the front driveshaft, the transfer case output shaft the pinion shaft should be parallel. While you can operate a standard driveshaft on an SOA YJ, it is not ideal nor recommended. Generally, a transfer case drop is used to lower the angle of the transfer case output shaft and the rear axle is shimmed about 2 degrees up to compensate for the change in the transfer case angle.

The best solution is to install a Slip Yoke Eliminator (SYE) in the transfer case and use a Constant Velocity (CV) driveshaft. When you run a CV driveshaft, you basically want the pinion shaft and driveshaft operating in the same angle (in practice, drop the pinion 2 degrees to compensate for the lifting of the pinion when torque is applied). On my first SOA setup, I used an SYE on the NP-231 and a CV. Today, I'm using an Atlas II which has essentially the same setup on the rear output. So I must essentially point my pinion at the CV. Below left is a picture of the NP-231 transfer case with the MIT SYE and the CV driveshaft. Below right is a picture of the pinion and driveshaft angle. Notice the pinion is pretty much inline with the driveshaft.
READ MORE http://www.dehesa4x4.com/osburn/tech/soa.htm

Additionally, the guy from Jedi.com has a super extensive write-up on doing an SOA conversion that includes a list of parts you will need. Check it out: http://www.jedi.com/obiwan/jeep/soa.html