There might be a easier way but the way i would do it. Mesure your shocks eye to eye fully compressed and fully extended. Flex your truck out on somthing to max travel, what ever you use ie: rock, fork lift, hi-lift, etc...
You will want to do it with the front in the air and then the rear in the air because you will get different results. Find the points between the frame and axle where the mesurements of your shock will fit.
That's all I can think of