I would look at your springs. Coil springs are rated for how much weight it takes to compress the spring one inch. So if you have 500 lbs springs, it takes 500 lbs on each of them to drop the rear one inch. If you are running sway bars the lighter springs will be less noticeable in the handling of the vehicle. You could try a different height spring also... Check with Afco or a racing specialty store and you can order the springs in different heights and by 25 lbs increments to tune it correctly. If you go to a shorter spring make sure it doesn't effect your shock travel too much. Fuel in the tank isn't going to adjust the height of the vehicle much. 15 gallons is only 105 lbs of fuel, not enough to push those springs down an inch.