Might want to take a quick look to see if you have a F44 optioned 1/2 ton before you do anything. If you don't, you're dealing with axles and brakes which are all 1/2 ton stock; throwing on a set of 3/4 ton leafs on won't give you the towing (let alone stopping) capability of a 3/4 ton. If you're lucky, your truck has the F44 option which consists of the 14 bolt 9.5" rear end which means it's a "heavy half" which is essentially a "light duty" 3/4 ton. If this is the case, adding an overload spring to the pack may well solve your problem. I opted for an additional leaf, Timbren Kit, and Bilstein HD shocks. Truck now squats 1-2" with a 1/2 ton load in the bed, handles much better than it did stock. If you have a 10 bolt rear axle, swapping to a 14 bolt (semi floating) along with the appropriate leafs (and the bigger brakes that come with the axle) is the best way to safely increase your towing capability.
Good Luck.