Warm it up for 6 miles. Then 'drive' it on the jack stands.
Use a LONG steel rod to listen to different parts/areas of the trans to narrow down the spot. A long rod like a dowel or even a floor jack handle. Hold it to your ear like an automotive stethoscope.
Maybe a bearing is coming apart.
Could very well be the rear main bearing, although it doesn't sound like a bearing at least to me.
I might deploy your steel rod method of detection, but I think I'd rather take the shifter boot off and "probe" through that hole with a smaller rod.
The thought of anyone getting under the car with anything while it is in gear and hauling just doesn't sit well with me.
I've just picked up a Moroso transmission "plug" (pipe stub with plastic cap on the end) that I'm told will fit in the output seal in lieu of the slip yoke. I might try to see if I can dry run the thing in the garage with that.
Perhaps eliminating the driveshaft/yoke from the process might prove illuminating?
