My '08 AWD has about 42k miles. Recently I pulled into the driveway after a freeway run of about 40 minutes @ 75MPH. The temperature was in the 60's. Thirty minutes later, I started the car and backed up. The car felt like the brakes were still on (they weren't). The engine was straining, but the car wasn't moving easily. Once in the street, I put it into drive with the same results, engine working hard but not moving very fast. I went slowly around the block with no change and no shifts occurring. There were also no warning lights showing a problem. Back in front of the house, I shifted into reverse and was treated to a loud bang (or wham). I put it back into drive and parked it for about 15 minutes. Before I shut it off, I checked the fluid, it was full, clean and smelled normal. After 15 minutes, I started the car. It was fine and has been so for the last 3 days. Any ideas?