Can somone please explain to me what these do and why they are reccommended if not required for certain lifts?

 

reduces angles and gets rid of the slide in yoke at the transmission helping to eliminate drive shaft vibrations

 

The drive shaft is connected to the output shaft on the transfer case with a slip yoke. This allows for some give when your drive shaft is forced forward when your rear axle goes over a bump. The SYE eliminates the slip yoke and replaces is with a fixed yoke. The "slip yoke" is moved to the drive shaft in this case.

These are recommended for larger lifts because of the driveline vibrations that some people get, like me. What causes this vibrating exactly, I don't honestly know.

 

Someone correct me if im wrong but the vibrations are caused because the farther the slip yoke is slid out the more play it has on the tcase shaft itself. That causes your driveshaft to be unbalanced when spinning.

 

http://www.jeepforum.com/forum/f11/o...l-read-426483/