Slip Yoke Eliminator kits and Constant Velocity Driveshafts:

What's a Slip Yoke anyways? A slip yoke is part of a mechanism made of two shafts, one shaft fits inside of the other, to create a single shaft that rotates together but can slip on itself to lengthen and shorten as needed.
In older Jeeps, this slip mechanism was placed in the transfer case and tied to the output yoke, this allowed the stock driveshaft to get closer or further from the transfer case as the axle moved. The problem is, this change in length could only happen in plane with the transfer case, which could lead to driveline angle issues.

Okay, what do Slip Yoke Eliminator Kits do? SYE kits replace the output shaft assembly of the transfer case with a solid non-slip shaft and yoke. This lets you correct for driveline angle issues, without worrying about the mount point moving.

What about CV Shafts? After installing an SYE kit, you end up with a rigid system with no room to change lengths. To fight this, a Sliding Spline, or Slip, Driveshaft is installed in place of the stock shaft. Slip shafts are two-piece driveshafts joined in the middle with splines, using the same slip yoke mechanism as the slip yoke in the transfer case, but moving the "slip" to the driveshaft allows the length change to happen in the plane of the shaft, which preserves angles and mounting points.

The Constant Velocity part of CV Driveshaft comes from the joints used at the end of the shaft to connect it to the other components. A CV joint uses two U-Joints put together, or a specialty CV joint to ensure that the components on either side of the joint are rotating at the same speed, or, a Constant Velocity.

Are there any other benefits? Beyond correcting the driveline geometry and allowing the system to expand and contract in the correct places, SYE kits will also seal the transfer case on the solid output shaft. This allows for a damaged rear driveshaft to be removed, and the Jeep can be driven under power of the front axle alone without any transfer case fluid leaking.

What about newer Jeeps? Many newer Jeeps, like the JK, come with slip driveshafts instead of slip yokes in the transfer case, so there's nothing we need to change.

Are there any other options? Upgrading the transfer case is a popular alternative, and can grant you new gear ratios or drive options. For example, this is what we did for Stubby instead of an SYE, we went to an Atlas II transfer case, which gives us not only the benefits of an SYE, but lower gears, a much more robust transfer case, and the ability to control the front and rear axles independently. This is pricier though, and isn't worth it for everyone.