It depends. I didn't "need" the SYE and CV shaft with my 4" lift, but I got good at swapping out the rear u-joints every 3 weeks. I did have a 1" t-case drop on it at the time. Now with the SOA on 1.5" springs I did need the SYE. Once you install the kit, you can always jack rear end off of the ground and support the frame with jackstands, to let the rear end flex all the way out. Then see if you can spin the driveline or not. If not, then I would do the SYE. Since I drive mine to and from the trails, I like the fact that if I do break a rear driveline, I can still drive out in front wheel drive without puking all the ATF out of the t-case.