'97 Ram 1500 bouncing off the asphalt!!!!!

Scared to death. I was crusing down the freeway at 65, accelerated to pass a semi and as soon as I hit 80 MPH the back end of the truck started bouncing up and down savagely, and almost gave me a heart atteack!!! It was almost as if the ABS on the drum brakes had kicked in. I was in OD, RWD not 4x4 and it was 14 degrees outside. I hit the brakes and slowed down fast and pulled over to the emergency lane and put it in neutral and then tried to calm down. I left it running, pulled back into traffic and never went above 70 for the rest of the trip. Holy s&*t I was scared. Does anyone have any idea what that might have been? Am I going to die on the freeway anytime soon? How come it drives normally at 65 to 70? Some advice would be greatly appreciated otherwise I'm getting rid of it very quickly. It's a 97 with 117K miles on it, transmission fluid and oil changed religiously.

Robert.

check the oil in the rear lately??

Am I going to die on the freeway anytime soon?
You just might if you keep doing 80 everywhere.

What kind of tires are you driving on. I had a set go "out of round" on me
At about 75 it felt as if the rear end was going to come off.