Upper StableLoads will help with your rear squat. The shocks will help dampen movement reducing sway or porpoising but will not prevent sag. If the the upper StableLoads result in too harsh of a ride when unload, consider using Energy Suspension bump stops instead but realize that they will compress to half their height before engaging the overload spring.
On my F250, I tried both and even ran a mix of SL and ES stops to find the right balance between unloaded and loaded ride. I did find the ride better with ES stops, but needed the full the support of the SL stops for the weight I was carrying (7000 lbs on rear axle). I did have Firestone airbags to help leveling out, but only need them when towing my trailer (8000 lbs on rear axle) once I went with the upper AND lower StableLoads. Anything less than using the full complement of SL's required help from the bags to minimize sag.