Well, to add a little to what's been stated already.
You're transfer case has a slip yoke on the rear. The rear driveshaft slips in and out of the transfer case as the rear axle moves up and down. The tailhousing on the transfer case is also longer than the SYE, so by getting the SYE you can get a longer driveshaft which means less of an angle.
A BIG advantage of the SYE, besides a better driveline angle, is you can remove the rear driveshaft without losing all of your fluid. Once you "slip" the stock driveshaft out of the transfer case, all the fluid comes out. The slip yoke of the driveshaft "seals" the transfer case.
Like Cynic said, there's no way to tell if you'll need one or not unless you put the lift on. Should you? Yes, but that's decision you'll have to make according to your funds. Setting your rear pinion angle will depend on whether or not you have one or not. HTH
Jeep with stuff.