You can't get rid of the stiff ride, but the control and stability you are after will be found with bilstein shocks. Hands down, no question, they are the best I've ridden on in the Xterra. They won't take away the bumps, but they will help keep the ride more level and in better control.
I believe the Tokico shocks are low pressure, so they should actually be quite forgiving. I have not tried them though.
I did have the Rancho RSX shocks in my truck and they are not stiff at all.
You'd have to make more substantial changes in the suspension to improve the ride significantly. Solid axles, leaf springs, and torsion bars attached to a ladder frame are not ride friendly by any stretch of the imagination. Infact, considering the underpinnings, it's quite an accomplishment for Nissan to make the truck ride as good as it does. Shocks will only help control the body movement after you've hit a bump and will help your truck rebound better.
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ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy