Pulsating vibration while cruising??

I had a similar problem with my Dart (although with a 7 1/4 rear end). The pulsing got worse at higher speeds. It turned out to be a bad bearing at the rear axle. I will admit that I also had a bad u-joint in the drive shaft that seemed similar. I could feel that vibration in the hump down the middle. I checked that by throwing the car in neutral and coasting while driving. The drive shaft vibration would go away in neutral. Those are the two places I would start.