Asking for input/help diagnose, not an a-body

Dana67Dart said

Sounds like dead rear shocks.

Get the rear bouncing up and down then let go. If it continuous for more than 1 cycle. That's where I would start.

You could also ride behind as she drives over the offending speed and location and see what the suspension does.

Also look for loose warn or broken rear suspension parts.

Click to expand...

Its not an up and down movement, its left and right and doesn't happen at lower speeds. I'm wondering if it has some type of posi' traction in that rear that is shifting the torque from one axle to the other.
Rain ended but its gonna take a day or so to dry up out there. We'll get it up on stands and see what we can finger out.
I was hoping someone had seen this before.