[joedoh]

you dont need bigger bags in the back, all the weight is in the front. It was said above that the bigger the bag the larger the volume and the lower the psi, that is absolutely true, but apply that truth to a larger bag in the rear and think about it. You will have full lift at about 20-30 psi, and no spring rate to speak of, even with shocks it will behave like the softest sprung car ever made. Thats cool for highway cruising, but it will really work out your shocks on a bouncy road or over potholes. At ride height you will be at 10-15psi in the rear and it will be jouncing and juddering all over the place.

You need a bigger bag up front, because the much heavier weight will give a higher spring rate on a larger bag and that will translate to a good ride, too small a bag up front would be very stiff. Even the smallest firestone bag out back will handle most any load you can put in the bed. Most guys with bagged trucks dont haul anything heavier than a couple chairs and coolers anyway.

if you are set doing bags behind the axle, I would suggest you use firestone sleeve bags. Make sure you put on shocks to limit their height but they will give you 8-9inches of lift and ride so smoothly at ride height. Sleeve bags will not work on a leverage setup where the bag is on the link bar.

edit: make sure you are planning to run shocks with bags. they are absolutely necessary.