The rear DS won't move that much with it being in the middle so a longer slip joint isn't needed.
A CV joint might be good if you got one hanging around or can get a hold of one for cheap, otherwise you might be able to get away without it depending on how much lift you have and the angle of the DS.
Also here's a writeup I did on the sq. DS. Less pics then the one by MiniSimp, but a little more wording. Maybe the two of them together could make the perfect writeup.
http://www.toyotaoffroad.com/Articles/Projects/Square_Driveline/Square_Driveline.htm
