Burning smell coming from the left rear brake

[twilkinson]

Twice now my 2006 F250 started to vibrate severely in the rear end going down the highway. I pulled over and noticed a burning smell coming from the left rear brake caliper and disc. There was an extreme amount of heat from the brake dragging. I turned the truck off, sat and waited for 30 minutes, the brake cooled and I drove off the first time, no problems until now. This time I could feel the brake grabbing on the disc (probably warped from the extreme heat) at idle speed. I waited once again, drove off and after a day, still no more issue. Any ideas on what could be causing only the left rear to suddenly grab and start dragging causing this severe vibration?