grease slinging on floorpan
It sure sounds like a U-joint problem about to happen, the vibration could have been caused by the joint and when it heated up the grease exited where it was supposed to be. I'd pull the drive shaft and take apart the joint and look then you'll know. Hopefully it was just an excess of grease, but better to know for sure. You'll be able to see the wear easily on the cross.