The reason that it can't be used in the ATX is simple - its a different transmission. Same reason the part can't be used in the Getrag 6-speed, or in a Taurus. Each model of transmission is pretty unique, the only case I've heard of that used the same differential for both manual and automatic transaxles was in the Neon (and JA sedans). More typically, they're supplied by separate companies, designed by different groups of people, so they have little in common...
