When I purchased RS9000's for my last rig, I removed the existing shocks, used a floor jack to fully compress one side and measured the length from eye to eye (gives you compressed length when on the bump stops). You will want a shock that is a little shorter than this measurement compressed to keep the shock from bottoming out (also consider bump stop compression). Then measure at full drop for full extension length. The optimum shock will have a fully compressed length an inch or so shorter than your compressed travel and an inch or so longer than your full drop. This will keep the shock alive MUCH longer. Once you get these, just go to your local parts place, tell them the measurements you have and what kind of upper and lower mount you have for the shocks, and they can find out what you have... If your doing something like the Rancho's, they have a chart on line that shows the shock lengths and mount types..
HTH,
Zig