ranchos longest shock is a 5012 or 9012, the 5000 or 9000 series.
it has a 13.5" stroke, which if placed correctly on the axle tube will allow for plenty of flex. mount the shock near the center of the stroke for up and down travel.
total extended length of the shock including the body is 34", thats almost 3 feet of shock under your rig, remember when calculating not to allow the springs to flex in a negative arch, it will breakdown your springs (use bumpstops)