Yes you will need it. I have a 3.5 RE lift and drove around for about a year w/o the SYE kit and a longer drive shaft. The torque from the I6 will make a bit of a shake between 0-10 mph. Nothing much to worry about if you want to mess up your tranny. I got lucky, I did have a 1" Tranny drop. But just recently got rid of it and gained an inch of clearance. Tranny is fine for now.
I'd suggest to save some money, if you can remove the drive shaft and take it to some steel/fab place and ask them if they'd do work on your drive shaft and add 3-4 inches to it. I'd ask the pro's and see what the exact measurement is. My drive shaft was 11" and i needed an additional 3 - 4 inches. Had a custom one made, and cost me a good chunk of change. Probalby would have saved around 250 bucks to take my stock one there and have someone lenghten it for me. Also ask around some local jeep/4x4 forums in your area, there are plenty of guys out there that do their own work or work at a shop that can hook ya up.
Hope this helps