Using the spring bars, try to get the trucks front axle to where the manual says it should be. I always try to get mine back to the unloaded weight so my headlights aim correctly. Many vehicle manufacturers only want the front axle to be only half of the unloaded weight though.
Once you get the front right, don't worry about the rear axle. As long as you are not overweight on it you are ok. The rear axle will take care of itself if the front is correct.
Take a long look at
this thread for some help. :)
Barney