As Im sure your aware. With the coil over you have a spring and a damper. By using the coil over in place of the shock on a stock rear end you now have 2 springs and a significantly high spring rate.

For the coil over to be effective the diff needs to be able to move up and down. Hense the 4 link setup. Even with a very soft leaf spring arrangement I doubt the coil over will be 100% effective. And say you only ran the large leaf on the rear, You would be placing that spring under considerable strain and possibly fatigue. As that leaf is holding the whole axle assembley in position.

This ute is turning into a serious weapon