A supplemental suspension system for a vehicle having a rear axle and a leaf spring suspension system including a pair of leaf springs mounted over respective ends of the rear axle, the supplemental suspension system including a lower support member which is positioned above the rear axle, which extends between respective leaf springs of the pair of leaf springs, and which is fixedly mounted to respective leaf springs of the pair of leaf springs; and a plurality of spring assemblies which are mounted above the rear axle, each spring assembly of the plurality of spring assemblies being composed of, in combination, a coil spring and an air spring arranged around a common central axis and attached by a retainer, wherein the plurality of spring assemblies are supported by the lower support member.