ok to answer most of your questions;
the "chevy lean" is a common problem with most s10-sonoma trucks. 1st and 2nd gen models. The uneven stance is based within the front suspension, but I have not heard the exact cause.
It is more noticeable if the truck has been statically lowered. (not bags) It can be corrected easily buy installing a cushion under the coil on the lower side. It is also true that beltech offers this, I dont know the part ##.
As for the sagging of the rear of the truck. It is usually caused by frequently overloading the carring capacity of the truck. the added weight puts strain on the leaf springs and can flatted or "de-arch" them. It can also be caused by always having extra, but not excessive, weight in or on the bed. such as a canopy.
This also can eventually de-arch the leaf springs.
Probably the best way to correct the sagging is to add an extra leaf in the spring pack. It is fairly quick and easy to do. If you plan on carrying heavy loads on a regular basis, you should consider adding a leaf to the system. It will give the extra support for the rear of the truck, as well as restore normal handling characteristics, which can be compromised under a heavy load.
As for the different suspension packages offered by GM, I know there are a few different ones available. However, the ZQ8 suspension package is the one that is the "sport" or "lowered" package. I think it is around 1.5" lower in front and 2" in back. It also includes a larger diameter front swaybar and also a rear swaybar.