A SYE is a slip yoke eliminator that gets installed into the transfer case. It is installed to allow the use of a different type of rear driveshaft that is properly designed to eliminate the vibrations typically caused by 4" or taller suspension lifts. This is not to say that it would be inappropriate to install a SYE and DC driveshaft for shorter suspension lifts but certainly by 4" it should be considered a must for a short wheelbase TJ.
The factory rear driveshaft is called a single-cardan and it has two u-joints, one at the front and one at the rear. Its u-joints are not able to work in the steepened angles caused by such a tall suspension lift. Those steepened angles the u-joints are presented with are excessive with taller suspension lifts and they vibrate because of those excessive angles.
This is what the factory installed and your 4" suspension lift is too tall for the two u-joints on the rear factory driveshaft. The taller the lift the steeper the angles are for its two u-joints. When that angle gets excessive the u-joints start vibrating.
The type of aftermarket driveshaft that can be installed after installing the SYE is called a double-cardan. A double-cardan (DC) joint like shown below is also known as a CV (constant velocity) as the below drawing calls it. A DC driveshaft has three u-joints, two up front and one in the rear. The way it's designed eliminates the steep angles the u-joints work into so they don't vibrate.
This 4" height mentioned as when a SYE/DC driveshaft is needed is primarily for the short wheelbase TJ. The longer wheelbase Unlimited can go a bit taller before it needs a SYE and DC driveshaft due to the reduced angles at its longer rear driveshaft.
Something else that has to be replaced are the rear axle's fixed-length control arms. They hold and position the rear axle and need to be change to adjustable length designs. That's so the rear axle's input shaft, called its pinion shaft, can be raised so it's in-line with the driveshaft as this illustration shows. Shortening the upper control arms and lengthening the lower arms rolls the axle so its input shaft (its input shaft) gets angled/raised upward so it's at the same angle as the driveshaft. The rear u-joint is pointed straight so it has no angle to work into. The front two u-joints divide the angle in half so each u-joint is only working into half of the total angle.
In closing, some unscrupulous 4x4 shops will install tall suspension lifts like your 4" and "forget" to tell you that you'll also need a SYE and DC driveshaft to be installed. They "forget" to tell you because that adds enough cost to it that many would decide against going that tall due to the cost. They know the customer will have to come back later about the vibrations and be forced to fork out the extra $$$ for them to be installed to get rid of the vibrations. I hate shops like that.
The factory rear driveshaft is called a single-cardan and it has two u-joints, one at the front and one at the rear. Its u-joints are not able to work in the steepened angles caused by such a tall suspension lift. Those steepened angles the u-joints are presented with are excessive with taller suspension lifts and they vibrate because of those excessive angles.
This is what the factory installed and your 4" suspension lift is too tall for the two u-joints on the rear factory driveshaft. The taller the lift the steeper the angles are for its two u-joints. When that angle gets excessive the u-joints start vibrating.
The type of aftermarket driveshaft that can be installed after installing the SYE is called a double-cardan. A double-cardan (DC) joint like shown below is also known as a CV (constant velocity) as the below drawing calls it. A DC driveshaft has three u-joints, two up front and one in the rear. The way it's designed eliminates the steep angles the u-joints work into so they don't vibrate.
This 4" height mentioned as when a SYE/DC driveshaft is needed is primarily for the short wheelbase TJ. The longer wheelbase Unlimited can go a bit taller before it needs a SYE and DC driveshaft due to the reduced angles at its longer rear driveshaft.
Something else that has to be replaced are the rear axle's fixed-length control arms. They hold and position the rear axle and need to be change to adjustable length designs. That's so the rear axle's input shaft, called its pinion shaft, can be raised so it's in-line with the driveshaft as this illustration shows. Shortening the upper control arms and lengthening the lower arms rolls the axle so its input shaft (its input shaft) gets angled/raised upward so it's at the same angle as the driveshaft. The rear u-joint is pointed straight so it has no angle to work into. The front two u-joints divide the angle in half so each u-joint is only working into half of the total angle.
In closing, some unscrupulous 4x4 shops will install tall suspension lifts like your 4" and "forget" to tell you that you'll also need a SYE and DC driveshaft to be installed. They "forget" to tell you because that adds enough cost to it that many would decide against going that tall due to the cost. They know the customer will have to come back later about the vibrations and be forced to fork out the extra $$$ for them to be installed to get rid of the vibrations. I hate shops like that.
