the amount of 'lift' you get from a mono-tube shock is a function of the piston rod area multiplied by the gas pressure.
In a Bilstein, IIRC, they run approx 300psi, and the rod diameter is 14mm = 0.551" therefore the area of the rod is 0.238 sq. in. x 300 psi = 71.4 lbs of 'lifting' force.
The hard part to understand is that it dosen't act as a 'spring'.
By raising the vehicle, the 'spring' forces cancel out, and so no diffence is felt in ride, although the gas pressure does increase the forces needed to get the shock moving in 'bump', as the shock bump valving is on the piston. If the bump valving is totally housed in a remote canister, as in a very few race shocks, this 'nose' pressure is vastly reduced to basically zero.
Where it acts as spring pre-load is when the spring is trapped, as in some race cars that run 'nil droop'. If you add gas pressure here, you definately increase the suspension pre-load, and hence ride rate.
Regarding the inconsistent ride height and a spring sagging, occaisonally some get through with incorrect heat treatment, and can sag before their time.
Take both left and right out and measure their free length, then compare it to the factory spec.
Springs sagging is very common here in 4 wheel drives that do a lot of outback touring as they are usually very heavily loaded.
Land Rover until recently, specced a 0.5" taller spring on the drivers side in all their vehicles. This had a totally different part number, although the spring rate is the same both sides.
I'm not suggestin scooby doos are like this 'though.