When you're on roads like that, disconnect the sway bars and let some air out of the tires (especially since you probably have E rated tires).
Lower the torsion bars, make sure all the components up front are in good shape, and consider some bilstein shocks. The torsion bars cranked up are probably the #1 source of the rough ride and new rear springs aren't going to fix that. You could spend thousands on the rear end and it will still ride like crap with the torsion bars cranked up.
My ranger handled washboard gravel roads AWESOME with bilstein shocks, disconnected sway bars, rear shackles and 15-20psi.
2006 F150 FX4 Super Crew - Bilstien 5100s leveled 2", 35" KM2s, 4.56 gears, Detroit locker, Eaton E-Locker, Traction Bars, AEM intake, Xcal 2
2010 VW Golf TDI 6MT