Mechanic Q: Clunk noise (which I can also feel) coming from rear of Lexus when I brake, what could be the proble?

Hi, I have a 2003 Lexus GX470. When I brake and come to a stop and then release the brake pedal, I get some kind of clunk noise from the rear of the vehicle (which I can also feel); however, if I brake and come to a stop very slowly I don't get the clunk noise or feel. I started to feel this not to long after I had a wheel alignment and tire rotation done. What could be the problem and how much could this run?