On a live axle application (for example, Jeeps where the rear axle moves freely when you go over speed bumps, etc because it is held with springs). When this happens the driveshaft needs to be able to get longer or shorter depending on the position of the axle. On a Jeep for example, this is where the slip yoke comes into play. The yoke is the part that will hold the u-joint and the "slip" is the splined portion that slides in and out of the transfer case.
The SYE (fixed yoke) eliminates the slip yoke. The purpose is to allow you to install a CV(or double cardan) drive shaft like the one pictured below.
Why do this? Well it lowers the angle and removes vibrations that arise when you lift a vehicle. Another advantage to this is that the rear of the transfer case is now sealed. Meaning if you break your rear drive shaft, you can drive home in front wheel drive without spilling transfercase or transmission fluid all over.
If you have vibrations on your rear driveshaft, you can sometimes figure out what is causing them depending on when they occur. Vibrations caused by acceleration usually mean that the pinion is pointed too high. Vibrations when you let off the gas usually mean too low. Vibrations during acceleration and when you are deceleratio, but not coasting could mean you have a problem with axle wrap. These occurances aren't always a set thing though, it's just something to go by.
edit: greg can't spell cardan
Edited by gregmondro, 31 October 2006 - 01:32 PM.