I put some helper springs I got from jcwhitney on my 98 f150 and it made a world of difference with stiffness. If I stand on the tailgate and bounce the truck it barely moves now when before it was pretty awful. And while driving I can feel less body role in the rear, but its not to stiff at all. They were only 50 bucks so I would recommend them to anyone out there who just wants to stiffen there ride. They also added about an inch to the rear end.

http://www.jcwhitney.com/autoparts/P...HELPER+SPRINGS