He had returned to the pits to have a vibration in all gears checked out--they jacked up the car and turned the left wheel while looking under the car for driveshaft turning--

I think(from past experiance) that the differential carrier (which Nascar guys occasionally lighten)(and I have seen unlightened ones break) probably broke and after a short while a spur gears cross shaft came out locking up the rear causing the uncontrolled slide---And I have seen in on GM stuff also, altho all the Nascar stuff is Ford 9 inch