Generally, with a rear bar you want to start on the softest setting if you have a choice. The harder you set it the more it will tend to cause oversteer.
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I'll second this. I just did energy suspension on the front, and will be tackling the rear shortly. Before I aligned it, the truck was borderline dangerous to be driving. And the rear can very much feel like it is swaying about if the front alignment is off. I would look seriously at the rear bushings in the leaf springs and shackles. If these are terribly worn, there would be some lateral movement. More than it should. The reason leaf spring vehicles don't have a panhard, generally speaking, is because it doesn't need one to keep the axle from shifting to the side.A loose and squirrly front could give the perception of a loose rear.