Why did my brakes begin to vibrate after rear lugs were replaced?

Last year, a local dealership replaced all of my rear wheel lugs (after a tire center had ruined several of them). Shortly afterward, I noticed a subtle but intermittent pulsing or vibration whenever I applied the brakes. I suspected the front brakes because I hadn't turned the rotors on the previous brake job, so I replaced the rotors and pads, but the problem did not go away. Now the pulsing/shuddering is pretty rough and happens almost always. I suspect that the rear brake drums were reinstalled sloppily somehow when the rear lugs were replaced. Any suggestions?