I assume that by "truck suspension" you mean parallel leaf springs. Those have been gone from the Mustang since the Fox bodied cars hit the streets in 1979.

There was talk of an IRS for the S197 ('05 to present) Mustang, but they elected to hold down costs. As solid rear axle cars go, the setup on the Mustangs of the last 17 or so years is about as good as they get.

As far as this coming from Volvo, the hot rodders were doing a four bar link in the fifties. Maybe Volvo got it from the hot rodders.

Have a great day,