I have a 2004 F53 20,500lb chassis. I put the KONI FSD shocks on all the way around and they made a HUGE improvement in the ride.
I bought them here:
It looks like the same shock fits the rear on all the F53 chassis' from 1998 on.
Koni Frequency Selective Damping (FSD) shock absorbers provide superior handling characteristics without compromising ride comfort. The FSD is firm where you need it, such as through corners, but rides soft over expansion joints and rough roads. A special valve filters out annoying vibrations before they enter the vehicle, resulting in a smooth ride over a variety of road surfaces.
Ford F53 Rear (1998-Current)