Clunking in rear when slowing down

Unfortunately it's impossible to know what could are happening without inspecting hands on. If "greasing" something helps, then perhaps find out further what is bering greased and why it helps. Sometimes, things like driveline angle can cause noise and vibration, but is adjusted as it is due to typical load carrying -- and an unladen vehicle can cause abnormalities. This is all guessing, but what comes to mind. Ask more questions of the folks working on the vehicle and learn first hand what is being done and why it seems to help.