Extra leaf would be my last choice, however another leaf will support more weight. It'll also make it ride rougher and look funny if the front suspension is untouched. Extra rear leaf could net you 2 inches or more lift in the rear.
How much is it squatting now? If your hitch is right it should be about perfect. What size weight bars are you using? Some rear squat is OK. I allow mine to go down a couple inches for a better ride, even though I can get it perfectly level with the hitch.
Other than that look at Timbrens.