i have a 1993 f150 and when im doing 55mph and step on the brakes the truck vibrates and when i get to about 35 mph while braking the truck vibrates worse and after i slow down past 30mph the truck doesnt vibrate. i changed the front pads and the rotors dont look bad. i also changed the rear brake shoes and all the springs and the brake drums. also the bed of the truck shakes really bad when im braking and i dont feel anything in the steering wheel just the back of the truck shakes. somebody please help i cant figure out what is causing the problem

 

You should pull the drums and check them with a dial indicator to see if they are warped or defective. You said you installed new drums right? Even though they are new they could still be out of spec.

You might also want to check the run out with a dial indicator on the front rotors to see if those check out.