Truck shaking or bouncing in the rear.

My truck just started doing this shaking really bad in the front, so I thought it was the front hubs or wheel bearings, I had a mech look at it and confirmed not the wheel bearings. He thought due to how low my tire tread was that I was having a tire come off my wheel. I had new tires put on thinking it was a tire problem but its back, in the rear bouncing and weirdly the brake pedal is stiff as a rock and the it acts as if the brakes are applied because it is hard to move the truck both while in cruise speed or at a stop. Please help.