If you don't want any lift, but you want to stiffen the rear suspension a little, there are two feasible options, and one ridiculous option.
First, you can get helper springs, adjustable, and they work. It's a basic system with adjustability that is easy to install, but there is labor involved in the adjustability, as you have to crawl under the truck and loosen bolts to do it.
http://www.amazon.com/Hellwig-550-L...RUWU/ref=sr_1_2?ie=UTF8&qid=1331871449&sr=8-2
Second, you can run air bags in the rear. Although not a practical application for off road usage (due to overstretching bags), the air bag system will ride good on the road, and it has adjustability that doesn't require tools to adjust. Compressed air is all that it takes to adjust the air bags up and down. Installation is more difficult than helper springs.
http://www.amazon.com/Firestone-W21..._2?s=automotive&ie=UTF8&qid=1331871640&sr=1-2
The third option is ridiculous, simply for the cost of it, and due to the fact that the problem will come back within a few thousand miles. You can replace the leaf springs with the same ones that came on the truck. Installation will be difficult and time consuming.
http://www.courtesyparts.com/kb_search_result.php?keywords=[55020R]+\(N50&cPath=533_534_537_545
So basically unless you tow a lot it works like this. If you want to beef up the rear suspension (which you do). Your best option is also your cheapest option in the form of the Helper Spring. It's easy to install, and once you get it adjusted properly, you don't have to change it until the load on the rear of the vehicle is significantly different. Your second best option is the air springs. Not good for off road, but good on the road, they come in at a middle of the road cost. Your worst option is purchasing new rear leaves. Not only will they not fix the problem, they are also the most expensive of the bunch. Good luck!