Drive train vibration. Scratching my head.

More follow-up. Looks like the vibration was from a combination of bad u-joints and transmission mount. I still get some vibration above 80 (no comments). I was speaking to a mechanic friend of mine and I remembered that years ago I had removed a funky looking bracket and weight (weight savings) from the back of my trans. He said that was for vibration dampening. I found the bracket but tossed the rubber isolation weight. It was about the size of a large soup can. Anyone have an idea on fabricating one or other solution for dampening?