Contact Rovertracks about modifying your t-case to run in rear only and 4 locked. He can do it at what I remember was a reasonable price. Unfortunately, putting on selectable hubs will not net you anything. With the d-shaft, pinion and shafts all still spinning, there is almost no gain in performance and definently none in handling.
To your first question. A CV and a Double Cardan driveshaft is usually the same thing. I did just look at the SG site and that DS does look different. I don't know a thing about it though. This page is very informative if you are interested in a little more driveshaft tech:
With no lift, you should really not need one at all. If I were you, I would buy new u-joints for the one you have and take the entire assembly (ears too) to a shaft place and have them balance it. If that doesn't work you know it is not the shaft, and it should be no more than $100 max.
To question 2, these trucks just have a lot of slack in the drivetrain. It's almost inevitable. Although, it did decrease quite a bit with a spool in the rear and drivslugs up front, but don't do that!! Just something I noticed. Good luck!