Tires: shimmy with unidirectional tires, directional tires, alignment specs

Question
I have had two cars for which I have bought unidirectional tires.  As the tires get close to the end of their lives, both cars have developed shimmies when driving faster than 60 MPH.  I have always rotated the tires front to rear (never crossing) and the tires look to be worn evenly.  When I have replaced the tires, I have always had the alignment checked and have never had the alignment out of spec.

I have noticed on my 626, which has the shimmy now, that the right side tires appear more worn than the left side tires.  I am assuming it is this slight difference in tire diameter that is the cause of the shimmy.  Does this seem reasonable?  What could cause the uneven wear between the left and right side?  Is there any reason I could not have the left rear and right front tires unmounted and mounted (and spin balanced, of course) such that I can swap them with the "arrows" still pointed in the right direction and drive another 10,000 miles?  The tires are not worn down to their wear indicators yet and this would put the best two tires on the front.  

Answer
I think you have a slight misalignment problem.  Remember, "in spec"  could mean at the edges of the tolerance.  Plus there is a compromise in the steering geometry that can not be detected by looking at the normal alignment specs.  The compromise involves setting a car up to turn vs setting a car up to go in a straight line.  So if you do a lot of turning, your car will probably develop symptoms of irregular wear - a vibration being one of them.

But I am concerned about a couple of things.

Directional tires are generally directional to improve hydroplaning resistance.  As tires wear, hydroplaning resistance drops draatically and the effectiveness of the directionality is lost.

So if you are concerned enough to buy directional tires, why are you comprmising your safety by wringing that last few drops of tread life out of the tires?

BTW, conventional wisdom says you need to have your best tires on the rear to prevent your car from spinning out due to hydroplaning.  Losing traction at the front is less risky as slowing down will cause the tires to get traction, where a vehicle spinnng? - well it's too late!

Hope this helps.