Unstable steering

what tires?

what pressures?

what cruising speeds?

how many miles on her?

 

okay, maybe your struts are shot

 

+1 that on my older automobiles worn shocks or struts would cause an odd wandering left and right of the steering at highway speeds, so that would be one thing to check.
But could you also check for play in any of the linkage points in the front suspension?
Finally, a rare but possible source of left/right steering wandering could be slop or play in the actual steering mechanism—one car I owned had a worm gear type of steering mechanism that had play causing such wandering, but fortunately I found an adjustment mechanism that I could tighten to move the gears closer and take the play out. Rack-and-pinion steering mechanisms are less likely to have noticeable play but still possible. I do not have any such sterring problem and thus have not looked into the Prius steering system yet.

 

Jack the front up and check individual tire for play. Hold the tire at 3 & 9 o clock mark, rock it by push pull motion. Hold 12 & 6 o clock and repeat.

 

Have you ever had the wheel alignment checked? If not, I suggest you do that first.

Also, if you haven't had the front struts/rear shocks replaced that should be on the list before installation of new sway bars.

I've owned 4 Prius and I haven't noticed a problem with unstable steering with any of them. The Prius is not a BMW so don't expect that level of handling.

 

This sounds to me like underinflated rear tires and you get that motion on the highway. The other culprit may be tire rotation: Wen the tires get used to tuning in one direction you cannot cross them and have them rotating in the opposite direction without absurd handling problems. Changing the tires back to the original sides and rotational direction (no matter front or rear positions) will correct the issue.