Safari Van vibration when coasting/let off gas in rear of vehicle

2002 safari 2 wheel drive van, a strange clicking, rythmic knocking noise happens at the rear wheel area, sounds like it is in the center of the vehicle. whenever I am coasting, foot off of gas at all speeds. I lifted rear end up and pulled on wheels for any bearing problem, spun wheels in neutral, just the clicking of differential in the middle of the axle, no real noise or grinding sounds in the wheel hubs, what could it be? Drive line? U Joints? bearings? any ideas appreciated Brett