I have a 97 dodge ram 1500 4x4 , the rear end starts bouncing really bad when you drive it on a slight bumpy road going 45mph, you have to slow down to about 25mph to make it stop when it does start. you can all most can feel the rear end wants to rip off of something, but everything looks good to the naked eye under the body. what could it be. thanks for the help
Vibrations that severe can be caused by a couple of problems.
A frozen or locked up universal joint will do this at a certain speed.
Jack up the rear of the car and support with jack stands and chock the front wheels, put the car in drive and reach the speed that you feel the vibration. If you do feel the vibration then remove the rear part of the drive shaft and see if you can move the joint. If you can't move the rear joint then this will most likely be your problem.
Look at the rear tires to make sure the tire treads don't look like they have separated or look like they are uneven like having a bubble in the treads.
An out of balanced tire can also cause this.
Let us know if you need more assistance.