RE: There's a few things to check...
I've never seen a converted self leveling system, but I'd imagine that there's quite a bit to change out. These were taken from the Performance website:
<img src="http://assets.performanceproducts.com/assets/bitmaps/92.gif">
The Shock (4) is $275, and the air cell (3) is $105
<img src="http://assets.performanceproducts.com/assets/bitmaps/91.gif">
To eliminate the self leveling feature, I'd think you would need to replace (3) and (4) in the previous schematic with (1) and (6) in this picture (Spring, $70, and shock, $81)
Pros and con?
Usually the SLS sags because of worn air cells only. For $105 each, you preserve the SLS function (compensating for rear loading) and OEM ride quality/handling. You must also continue to maintain the entire leveling system.
With the conversion, $150 per side, you simplify function and maintenance by cutting out the mechanicals. You also lose the ability to compensate for hauling cargo, and may introduce handling and ride quality issues.
Shocks wear and need to be serviced, just like air cells. My car has shown that the SLS is a pretty robust system- the air cells were replaced ~5-7 years ago and since, the car rides like new, firm but compliant, and no sagging. If I were you, I'd get the system checked out and if everything else is still working, stick with OEM.