I need help with a new vortec max 4dr chevy. I am having two problems that could be connected. 1st when I load the front of the truck with a hard stop and then let off the brakes and move forward it thuds real hard in the rear end.It almost feels as if the rear disk are catching and then releasing, but i don't think it ios the brakes , if that makes any sense. 2nd when I am above 90 mph it feels as if the drive shaft is about to come apart I get a scary vibration in the seat of my pants. I need to some help diagnosing the problem.