Just look in the rancho catalog (
http://www.gorancho.com/products/docs/2008_Rancho_Catalog.pdf) at the stock length shocks and the types of fittings it has. Then just add 2" to it and keep the styles of the ends the same.
The f-150 mounts aren't quite 2" so if you get shocks that are 2" longer there is a chance you will bottom out the shocks on a hard bump. If that happens you may break off the lower shock mount (they make kits to fix that).
If your just trying to get a little more "droop" for what you have now, you can just put some washers (about 1" worth) on the top mount/stem under the upper shock mount. That will allow the shock to drop more and will keep the amount of travel the same. I don't think you can put too many washers on it to the point you will bottom out the shock since the upper stem isn't quite that long.
~Mark