High tire pressure causing shimmy on highway grooves?

So I finally took my new-in-late-May Prius out of town. On a long stretch of grooved highway, there was an unnerving shimmy. Not dangerous, but not fun. A couple of weeks ago, I was talking to a tire store owner (who is recommending the Nokian WRG2), and he inflated my stock tires to 44 psi, which is the max on the tire sidewall. He assured me that this would be just fine. Yes, the ride is rougher, but I'm not used to cushy rides anyway, so who cares. I do, however, think that the high pressure is related to the shimmy. (This based on the post in this forum with the link to tramlining.) So I'm going to back off the pressure to 42 front, 40 rear.

I'm hoping that the reduced pressure takes away the shimmy. That or maybe the problem will go away with a little tire wear?

Also, is there a resale market for my stock Bridgestone Ecopia tires? I know they are expensive, but those Nokian tires are calling my name. My previous car was a Subaru, and I'm a mite concerned about driving in the snow this winter.

 

Update: I just checked the tire pressure, and it was actually at about 40 psi. The tires must have been warmer than I thought when he inflated them. Or my gauge is crap. At any rate, I reduced the back to 38 psi. We'll see if that makes a difference. I don't want to go much lower than this pressure.

 

I can imagine it's much stranger on a motorcycle.