Softer rear suspension on a 2007 RC?

My FIL recently bought a 2007 RC Tundra after years of my wife and I extolling the virtues of Toyota over his preferred Chevy. The truck has very low mileage and was a great deal, but he is ready to give up on it after a month of so because the stiff rear suspension is turning him into a bobble head when he drives it on the back country roads around him.

Any thoughts or idea on how to soften up the ride on the rear end so he can drive it comfortably. He doesn’t do any super heavy hauling besides taking a load of 6-8 goats to market or bringing home a load of 10-12 hay bales from time to time.

Can we remove a leaf from the back end to soften things up or does anybody make shocks that will dampen the bounce from the rear end enough to quiet the ride? I am a complete novice when it comes to the suspensions on these trucks so any advice is appreciated.