I think the "coilovers" BTD was asking about are along these lines...
https://www.cachassisworks.com/images/VASCOid.jpg
These are not "load levelers"... They are a fully adjustable (either single or double) coilover shock which replaces either the stock coil spring or leaf spring to support the back of the vehicle.
A number of companies manufacture coilovers and not all of them are adjustable and spring rates play a large part in getting them right.
Mark