I don't really know much about it, but as I understand it, a Detroit makes it so neither axle can go slower than the ring gear. I don't _think_ it "locks" really. When you turn, the outside tire has to go faster, so if you're on the gas I think you end up driving the inside tire and the outside one coasts ahead. If you let off the gas it switches so that the outside tire ends up driving the driveshaft with car momentum since the ring gear is suddenly going slow relative to the outside axle. I believe this makes the clunk noise.
Someone who knows more can correct me. Anyway, I'm not sure how this might help with your problem. Maybe it is installed wrong, or maybe you are mis-reading normal quirks as being totally locked.