Take the dog/hostage out of the trunk....I keed I keed.
Since you have an open diff I am leaning towards wheel bearings going bad. I had a similar experience recently. Replaced rear wheel bearings and it went away. The rears are hard to diagnose because initially they seem normal until you release the torque off of the axles then you can hear one or both grind as you spin them by hand.
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-Retired
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