2008 Cadillac CTS drivetrain & axles problems

I was traveling at 25MPH when there was a sudden very loud explosive/grinding noise(it brought people running out of adjacent buildings). I hit my brakes immediately, but the noise continued until I put it into park. I exited the car and under the rear axle of my vehicle were multiple large bolts and screws lined up in a row. The vehicle was totally disabled, and subsequently was towed to the dealer. They determined that all the rear axle bolts just fell out at once.there was no other damage found to the vehicle.

I found a bolt laying in my driveway under the car. When I got in to drive it made a terrible noise as though something was loose. I took it to dealer and they said a bolt fell out of the driveshaft and the driveshaft needs replaced at a cost of approx. $900. I called GM to find out why a bolt would just fall out and why the driveshaft would need replaced with less than 38k miles and they could not answer me.