here is what the articlr from mustanggt.org says:
One big mechanical revision for 1984 involved the GT’s suspension which was introduced as a running change early in 1984. The rear control arm pivot points were lowered 1/2-inch on the upper end and 1/4-inch on the lower end for improved suspension geometry. In addition, quad shocks replaced the old-style traction bars and the rear anti-roll bar diameter increased to 0.79 inches from 0.67 inches in ‘83.
So they were available in 84'
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