Professorpavy,
Would you be referring to a 'two speed' rear end. This was a feature of the differential, not the transmission. It allowed for two ratios, the installed ratio of the ring/pinion, and through a set up small gears, a lower ratio could be selected.
This allowed an under drive or a direct drive to be selected, independent of what gear the transmission was in.
Earlier versions used a vacuum actuator, and modern versions use an electric motor. They are still used today.
Maybe this isn't what you were thinking of.
Stuart
