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As with any handling or suspension problem, you need to be sequential in the evaluation until you find the problem. My approach.
1) Check the tire pressures, if the rears are low you will get the sensation you are reporting. We are assuming you are running good quality tires. If you're running Kuhmo or something like that, then all bets are off.
2) If you are pushing and near the limit of adhesion, which I don't think is a good idea on public roads, and you are "coasting" you are unloading the rear wheels and adding more grip to the fronts, that will also create a yaw motion or what most of us would call oversteer. If you've ever come into a tight decreasing radius corner a bit hot, sometimes all you need to do is get off the gas to have the rear end come around until you lined up with the apex, apply throttle and then grip comes back to the rear and you've on your way.
3) Check alignment- If you have any toe out, the rear of the car will want to oversteer into corners. Quick example. Driving my friend's GT350 at Roebling road and normally this car has fantastic grip (running Cup2s), but every time I get into hardly any throttle at all in the corner the rear end steps out. Find out later, rear had toe out. This is the opposite of your situation (off throttle).
4) Worn out bushings- Maybe, and yes it's worth checking. I just replaced my rear subframe bushings at 142,000 miles and they looked great and this car has seen a lot of track use.
5) Any history of accident damage? Less likely but might be informative if something ever happened to the back.
6) Shocks are in good shape? Worn or poorly performing shocks will also create some uncomfortable and funny instability at speed.
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