Differential Unit problems of the 1996 Acura TL

I was headed up a hill when I pulled over to the side of the road to let an emergency vehicle pass. When I attempted to reenter traffic, there was a loud bang, some grinding noise, and a jolt as the vehicle began to roll backwards. I was startled when I immediately placed the vehicle in park but it continued to roll down hill. Then I engaged the parking brake, (I never use the parking brake due to an unfortunate happening in the past with a different vehicle). I attempted to put the vehicle in drive, thinking the transmission needed only to reset itself when a loud grinding noise began to emanate from the underside of the vehicle, along with smoke coming from my hood. At that point, I left the vehicle there and had it towed to a shop where I was informed that the differential had somehow become disengaged from the transmission.