Kind of too bad to have to stiffen the ride by adding leaves.
You might want to consider adding air bags to the rear of the truck so you will have the smooth ride and be able to adjust the capacity as needed.
I think the leaf would add maybe an inch of height permanently to the rear of the truck. A lot of people seem to be wanting to level their trucks by adding a leveling kit to the front.
Personally, I kind of like having a low-rider pickup and that's one of the things I like about it. I think Toyota made a step in the right direction when they went low-to-the-ground with the Tundra.
Good luck with you leaf addition if that's the way you decide to go. Too bad they didn't make a true 3/4 ton Tundra for situtations where the stock truck just won't get the job done.