I have a pair of Rancho 1500 LB overloads that I have used on two F-250s. Easy to install, they just fit over your existing leafs. Using a block over the axle and four really long U bolts, they pull up on and assist your sagging springs. Plus you can adjust them by tightening or loosening the U bolts.
A few points, like the others said, get weighed to make sure you don't exceed your GVWR. 10 plys are a must. And finally, when you have settled in on your normal weight load and your spring tension, have the front end aligned. Any time you change the ride height, it also affects the front end alignment.
