I have a 1996 GT. A few days ago, I was driving, and when I got up to around 50 mph, I felt a quick rumble that lasted for about a second from what felt like the rear. It felt almost as if I was driving over those divots on the side of the road, only I was right in the middle of the lane. A few days passed, and it happened again, at the exact same speed. I don't know too much about differentials, so I cannot diagnose the problem myself. Any suggestions?