What is an SYE or Tail Shaft Eliminator Kit?

SYE (Slip Yoke Eliminator)

Essentially, an an SYE kit shortens the over all length of the
NP231 Transfer case (used by all YJs) by around 4 inches.
This is important to anyone who installs a lift (except for
body lifts) of 3" or more because it allows you to use a
longer drvieshaft.	A longer driveshaft helps avoid the
steep driveshaft angles inherent in a tall lift.  Of course
this also necessitates that you get a new two-piece
driveshaft made which fits your rig.

An added benefit of the kit is that it allows you to remove
the rear driveshaft without the tranfer case fluid leaking
out (as would happen with a stock NP231 transfer case).
This is an important feature in that it allows you to limp
home in front-wheel drive if your rear drivetrain is damaged
but still capable of coasting.