This was in the sticky frequently asked questions. Pretty good info. Not sure if it helps but it'll give you more info on your decision.
SYE and CV Driveshaft
The topic of SOA and SYE and CV Driveshaft comes up way too much. The SYE (Slip Yoke Eliminator) is used to remove the slip yoke that come stock on all YJ Wranglers. The kit is typically used when a Jeep reaches a point where the driveshaft angles become too steep to operate without vibration. While the amount you can lift a Jeep without a SYE varies it is typically necessary at 2.5-3”. Some guys can run a 3.5” SUA lift without vibration others get vibs at 1.5”. The 2.5-3” area is a ball park. The amount of lift achieved with an SOA lift with stock springs is between 5 and 6 inches. Many times people use a transfer case drop to lower the transfer case about an inch to achieve better driveline angle and reduce vibration. A big problem with this is that even with the drop and the minimum amount of lift (~5”) the driveline angles are going to be similar to a Jeep with a 4” lift without a transfer case drop. So even with the drop the angle is still pretty severe.
There is another advantage to the SYE. If you decide to not use a traction bar and use a slip yoke with a transfer case drop then do not be surprised if the slip yoke slides all the way out of the transfer case. The pinion can rotate up enough to pull the driveshaft out of the transfer case. Because the transfer case design the way it is ATF will then spill out everywhere because the case is sealed around the yoke.
Trying to cut corners in this area is difficult. Some look to a “Hack and Tap” type SYE kit. This involves cutting the existing transfer case output shaft and using a flange adaptor to turn the output shaft into a fixed mounting location. This cost around the same amount as a full SYE because you need the adaptor which will seal the transfer case near the output shaft. This job is done by the slip yoke in a stock set-up. A TJ transfer case is sealed before the driveshaft and is cheaper than a set-up for a YJ. Another benefit of the full SYE set-up is the increased spline count in the in new transfer case output shaft.
The CV driveshaft is necessary to deal with the larger angles of the lift. Even if your driveshaft was long enough to use with the SYE there is no slip in the shaft. The shaft slides in and out to allow for changes in the distance between the pinion and the output shaft of the transfer case. This was accomplished in the old set-up through the slip yoke. Any change in distance the yoke would slide on the output shaft. Without slip when the Jeep flexes parts will break because there is no way to account for the change in distance.
There are a couple of ways to go about this. The first is to order a driveshaft from a vendor. Most likely they will want the distance between the yokes of the pinion and the transfer case. It comes in the mail and you bolt it on. The other option is to have a front 4WD XJ shaft cut down to the correct distance.
Standard SYE vs Hack and Tap