I don't think your Yj has any suspension lift kit installed so the SYE is not necessary. The SYE help solve the many geometry problems associated with the higher angles of the U-joints on the driveshaft and vibration problems when those joints are near to or exceeding their maximum angles. The SYE design is more like a front wheel drive CV joint than the traditional U-joint and can eliminate those problems.
Cost wise you're looking at a new seal of less than $10 vs an SYE kit plus labor of totally disassembling the transfer case to add the kit plus parts might be close to $800.
I would go back to the shop that put in the new seal and ask why it's still leaking. Wipe everything clean, maybe spray it with brake cleaner so it's dry, and a quick drive and try to positively identify where the leak is. Sometimes oil can travel a long way to end up at the back.