[Hal H]

It sounds like the lateral link (black lateral bar in picture where it attaches at hub with bolt and large silver washer) or the forward link may have broken - could have been a defective casting.

 

[Hal H]

That would be the attachment point of the cradle itself where it bolts to the body - unusual, but it might have been a defect in the manufacturing and the cradle split where the weld would be.

With all the folks that go to the track, I haven't heard of the IRS cradle breaking, but I think yours may have been a faulty assembly and just let go when the part fatigued and broke at the weak point.