A drive system for positioning any one of a plurality of toner developing modules relative to a photoconductor drum of an electrographic printer includes an indexing assembly having a chain drive mechanism. The chain drive mechanism includes a motor driven tow bar that is configured to releasably engage a tow hook mounted to a movable toner cart. The toner cart includes a developer rack that supports the plurality of developing modules. The drive system further includes a lift assembly that is configured to engage any one of developing modules and move it away from the rack to a developing position adjacent the photoconductor drum and back to the developer rack. The lift assembly includes a pair of lift arms, each of which is defined by a four bar linkage. The lift arms include hook assemblies that are configured to engage lift pins mounted on opposite ends of the developing modules. Each hook assembly includes a first centering member that supports a movable spring biased second centering member. The opposite ends of the modules are configured to engage first gaping surfaces of a pair of gaping cams mounted adjacent to the photoconductor drum. This engagement establishes the vertical gap clearance needed between the drum and module for the developing process. The opposite ends of the modules are further configured to engage second gaping surfaces of the gaping cams to define the developing position of the developing modules.