Rear end clunk turning left

Still doing it every now and then on sharp slow turns. When it's happening I'll look back at the tire, and it's acting as if the rear end is welded locked. Today I went for a burn and it did the loudest pop, bang , clank yet. ... I've lifted one side of the car of the ground(rear) so one tire was touching and one tire free to spin, put it in neutral and could not get the free tire to spin(slip) at all. My buddie says this isn't normal what are your thoughts.
Don't know a lot of history on rear. Got it from a local reputable used parts guru,