[dsraven]

personally I wouldn't trust that. if the front of the driveshaft comes out of the trans it can make a real mess under the truck and if the fuel tank is also under there it could be a really bad day for you.
like said though, do the full suspension travel and take dimensions on that yoke each time. then you will have a good idea of how much change you will need in the shaft length as well as if the shocks bottom before the suspension hits the jounce stops, etc. for leaf spring suspension also factor in a bit for spring wrap which can cause the pinion angle to change.