[HiFlow5 0]

Welcome to the board, we're to help!

At first, before I finished reading your post I was think that you bushing were shot in the control arms, but as I read on I have another theory.

A lot of times when you have a gear swap done, places recommend that you install and alum driveshaft. This helps eliminate any vibration and excess noise associated with a steeper gear.

Another area to note are the yokes on the driveshaft, if you car still has the orig. d/s from 66, or you replaced it with a used one, then chances are the yokes are shot!

And finally, if the pinion gear was not installed propperly and the backlash is off there will be a lot of play in the drive line. From what you said, You can only hear the noise when coasting, I think this might be your problem. As when you are on the gas you putting more power to the ground and the drive line is tight, but when you coast there's less tension in the drive line, cause things to slop around more.