rear braking
2 Answers
Take it to your favorite shop that has a lift and have the rear end inspected - brakes, axle, differential, and suspension components. Since it occurs when you brake, the likely suspect is worn brake components but have the whole part of the car inspected to prevent bad things from happening when you need the systems to work properly.
Maybe you have damaged suspension bushings or struts. What ever is causing the problem it is probably an unsafe condition. Have it checked out before driving it anymore.