You all are missing the point here. This ain't about whether the truck is big enough, it's about Billy's wife's back problems and based on his couple threads about it, it's a fairly acute situation.
Sway bars will only serve to generally stiffen an already stiff riding truck. And regardless of how sqishy the back end is, when loaded, the front suspension is just as stiff and jarring maybe more so , loaded. This doesn't mix with a bad back. At least in my experience.
Aside from suspension, id be thinking seats. A heated fully adjustable seat with 1 or more lumbar supports can make a difference.
Possibly think about upgrading the seats from a newer high end truck if you're able to do that.
Remember it won't help how the truck rides though.