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.
Yes, that is why the LONG ROD, or steel handle from the floor jack, keeps the person out from under the car.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?
