The SYE does exactly what it says... eliminates the slip yoke. That is, the driveshaft will be fixed instead of slipping in and out during articulation. The CV driveshaft has a pair of U joints on the transfer case end rather than just one. That way it can handle a steep angle when a lift as large as yours is installed on the jeep.
I have heard of people getting away without this, but I have a 6" lift too (soa) and I can't imagine driving my jeep without the SYE or CV Driveshaft... especially on the trail... that driveshaft has probably come close to slipping right out of your T-case.
You probably do need it, but 1300 bucks is a lot. Buy them yourself, install yourself.... under $600