Thanks for your reply.
My rear axle toe settings turned out to be right on the money. However, now that I've spent a Saturday studying the rear suspension on my Prius I've come to the conclusion that rear toe measurements are meaningless. Why do I say this?
First keep in mind that each rear wheel is allowed to pivot around Trailing Arm Bushing No. 3. Imagine this scenario. Put the Prius on a lift, grab the rear of the right rear tire and pull it out away from the car, now pull the rear of the left rear tire and also pull it out away from the car. Lower the Prius back to the ground and make rear axle alignment measurments (don't move the car before taking the measurements). In this scenario you should get a toe in rear axle measurement.
Now lift the car and push in the rear of both rear tires and lower back to the gound. Repeat the rear axle alignment measurements. This time you should get toe out rear axle measurements.
Since each rear wheel is allowed to independently pivot around their No. 3 Trailing Arm Bushing it is my opinion that any rear axle toe measurement is a waste of time.
Keep in mind that the rear wheels do not pivot a great deal. This is because the No. 2 Trailing Arm Bushing limits how much the Toe Control Link can pivot around the No. 3 Trailing Arm Bushing.
Now I can talk about my experience with Prius handling. On smooth pavement with no crosswinds the Prius is the best handling car I've ever owned. However, I live in Idaho where it is unusual to find a stretch of smooth pavement and it is also unusual to have a day without significant crosswinds. It is indeed a special moment when the pavement is smooth and there are no crosswinds.
The Prius does not handle nicely above 55 mph on interstate pavement that is lumpy with 18 wheeler ruts. I now believe I understand the source of this problem and it has nothing to do with alignment. The issue is that the lumps in the pavement induce lateral forces to the trailing arm bushings in the rear suspension causing the rear wheels to "steer" the car in the direction of the lateral force.
In my first interstate freeway experience I had a tough time maintaining control of my Prius. Here was my problem. I would feel the rear of the car begin to pull the car to one side. I would compensate by steering the other direction. I would suddenly be veering way too far the other direction. Sounds like oversteer? Yes, it does. The issue was that no corrective steering whatsoever was needed and so even the slightest correction with the steering wheel gives a sense of oversteer.
I decided to test this theory. I went back out to the rutted interstate where the speed limit is 75 MPH. I convinced myself that I would not use the steering wheel to correct for any side to side movement. I would instead focus on holding the steering wheel steady and keep it aimed at a fixed point in the distance ahead. I off course would correct if it became obvious that I was going to drift out of my lane.
Here is what I found. In the deeper 18 wheeler ruts the tail of Prius "wags" as you cruise along at 75 MPH. I did not use the steering wheel to correct and focused on holding the steering wheel steady and straight ahead. As soon as you get to a smoother piece of pavement the wagging stops.
I've had my Prius a little over two months now and I think I've finally learned how to drive it. I think this "wagging" may take a little while to get used to. However, I believe there is no cure short of redesigning the rear suspension that will fix this problem. The rear wheels are merely pivoting on Trailing Arm Bushing No. 3 in response to lateral forces. They are doing exactly what they are designed to do.
When driving a Prius at highway speed focus on holding the steering wheel steady and don't try to correct for side to side motion that is caused by crosswinds or 18 wheeler ruts. The car is "self correcting" when these events occur. Any corrections that are implemented with the steering wheel will feel like oversteer because by the time you react to correct the car has also reacted to correct. If the driver and the car are both correcting you get overcorrection. A constant experience of overcorrection is worse than the "wagging" experience. From now on I'll just let the tail wag.