The self-leveler will only come into play if the back end keeps dipping too low, for example a standard car with a bootfull of stuff will wallow on an undulating road, the self-leveler 'pumps up' to keep the back end level. It has an internal gas spring and a hydraulic stage that 'pumps up'. If you lift the car it will no longer contribute, that is why Disco's have harder rear springs (no self leveler) and lifted springs go much heavier on the rear.
In my opinion having harder springs does help reduce body roll on the road, at the expense of axle articulation obviously.
Now if your car sits tail-down when stationary the springs have probably relaxed. Check also that the car sits level side-to-side as they often don't on old cars. The later is exacerpated by the radius arm bushes which creep, so new springs don't always cure it.
Incidentally, some LHD Diesel models have a harder D/S spring at the rear to counteract the driver and engine, having 2 harder springs at the front already (sounds a bit of a bodge to me).
If you don't want to lift the car fit Police Spec springs as these have progressive rear coils to help with all the c**p the police carry all the time, I think a new set is only about £50 off the top of my head.
