A sye mostly shortens the transfercase and in effect makes your rear driveline longer, thus reducing the vibrations by allowing you to run a double cardian set up (allowing higher angles and also forcing you to rotate your axle up to get the corrrect driveline/axle orientation). Yes, a TC drop will allow for correction of vibes, however, it puts strain on the motor mounts and in higher lifts, it may not be enough to eliminate your vibrations. In some vehicles (not positive in the xj), a TC drop will have a reverse effect on the front driveshaft making it a slightly sharper angle and causing front vibes while eliminating the rear vibes. Also, an SYE does what it stands for (slip yoke eliminator) by eliminating the slip yoke at the rear of the transfercase. As you lift your vehicle, the distance from your TC and your differential becomes greater and it will pull your driveshaft out of the TC. So during full droop situations, it becomes dangerously close to falling out of the TC. An SYE eliminates that scary situation by using another type of driveshaft, one very similar to a front xj driveshaft, in that it it is a telescoping driveshaft with the joint in the middle of the shaft instead of at the end allowing a driveshaft with greater droop possiabilities than a slip yoke type shaft. Hope this helps.
