Go the suspension and chassis sub-forum and read every sticky post at the top by Ron Sutton.
Quick synopsis on sway bars - All sway bars are just a metal spring. Mass produced pre-bent swaybars are notoriously difficult to model ie determine their exact roll rate, so its experimentation or crap shoot or follow someone else's lead. The splined bars are easier to model and the manufacturers provide rate charts for various diameters and widths with the catch that each manufacturer has their own method of rating them. Speedway Mfg is the most prominent manufacturer of the splined swaybar so you can easily stick with them. Determine what width of bar will work including a 0-5" bend in the arms themselves (arm lenght also affects rate).
Now you have no idea what rate you want, but if you are going from nothign to something, you probably don't want to go to far, especially since more rear roll resistance can lead to too much oversteer. My ballpark recommendation is to determine as I mentioned above the width and attachment points that will work, then go to the speedway site and see what is available. I would recommend starting with a 1" diameter hollow bar and fairly thin wall. Thinner wall means less roll resistance. Try it for awhile, and then maybe experiment with something thicker (easy to swap in and out).
You can also drill multiple holes in the arms, and by shortening the arms (by moving to the shorter holes) you can get a sense of what a thicker wall roll bar will do and give yourself some finer adjustment. IOW build something, buy a fairly thin wall bar and start in the longest holes, adjust to the closer holes to see if it performs better. If it does, you might want to try moving up the wall thickness.
For a rear swaybar I cannot imagine you woul dwant to use anything larger in diameter than 1", but who knows I've never tuned a heavy AWD truck.
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