I have a 2003 Mercury Sable with 130,000 miles. Recently replaced the engine with a 72,000 mile engine. My car severely shakes when I need to brake hard, and sometimes when I am gradually braking. I looked at the brakes and the pads and rotors are both in great condition which is a bit confusing. My mom was the previous owner for the past three years, and she said she never had the front brakes redone, and there is no paperwork for that fix either. I was shocked to find the pads and rotors in solid condition, with the pads still having a lot of life, maybe over a centimeter of pad life left.
What could be causing my shaking? Maybe just needs to be aligned? Is this more serious than just needing new pads and rotors?