Ours has a very metallic clunk under the same conditions. U-joints seem OK. I believe it is play in the diff unit or worn axle splines or both. When we back off on the gas there is a definite whine which appears to come from the rear end so gear backlash is likely excessive.
I changed the diff oil, drove it doing lots of starts and stops, and no more clunk. The oil didn't look bad coming out. I should have kept the bottles to pour the old oil in to see how low it was. Hopefully that's it. I think I will change the front now.