What is "all-trac"

All-Trac was a proprietary full-time symmetric four wheel drive system used on a variety of Toyota badged models from 1988 to 2000. It was considered a revolutionary advance for four wheel drive automobiles into the mainstream consumer market and its electronic/vacuum controlled locking center differential was rare in a passenger car. The system originated in Japan under the GT-Four moniker in 1986, but was not released in the U.S. until 1988 under the All-Trac moniker.