Buy a pair of Gabriel Ultra Ryder HPG (High Pressure Gas) shocks...if they are still available. I put those on my 78 Mercury Zephyr I had (Mustang GT shocks actually), combined with their shocks for the front too...They made the ride a little bit more harsh than OEM, but the car rode flatter around corners, as well as didn't move much if you leaned on the bumpers while it was parked. This would be beneficial for your truck, to make the rear end stronger for load-bearing...