Your issue sounds like shocks to me; I had a similar issue with our Dodge recently. Since it was at 130K on the mileage, I replaced all of the shocks with Monroe HD Gas Filled shocks; the rear shocks were the "coil over" type rated at 1200# for the pair. This smoothed out my ride considerably, and actually rides better with new shocks.
The truck does not bounce around or stiffen the suspension as some people would suggest. The coil springs simply control the rebound better, and add a slightly higher load capability. The shocks are preset at the factory for 60/40 rebound (ride) and that will not change, just the amount of control over the movement changes. Before with our truck and trailer loaded at our "best" hitch position, the front end of the truck would sit with slightly over two inches of front end rise. Now, front end rise measures only one inch.
A suspension accessory like Timbrens, while performing as it is designed, does not have the "variable load absorbing" control that coil springs allow, or even air bags. And of all three, shocks are the most cost effective on the wallet; while at the same time giving you the results you are looking for.
I agree with a previous poster.....unloaded, the Timbrens are a "beater" on your ride quality.