Could it be this simple?
 

I've always had a slight shimmy in the steering wheel at speeds from the high 70 to mid 80 mph range. I've got both 17 inch and 18 inch sets of wheels, both with stock sized Michelin N3 tires. Two weekends ago I put my 18 inch wheels on for upcoming HPDEs. Now the shimmy is gone, but I didn't have the wheels rebalanced. The only thing I did different when putting on the 18 inch wheels was to take off the wheel lock lug bolts and put on regular lug bolts. I'm going to have to weigh a lug bolt and a lug lock to see how far apart they are in weight. Could the wheel locks been a source of shimmy all these years?

It's possible, but the lugs would have a small effect on the wheel balance because they are near the center of the wheel.

It's more likely that you had some buildup on the hub behind the wheel or something that would make it not sit 100% flush and it got knocked off during the change. Lugnuts are definitely too close to the center to make much difference.

Thanks I was thinking the lug bolts are too centered to matter. The other thing I had done was the installation of an underdrive pulley. Could the power steering pump spinning slower have any impact? Whatever it it was, I'm going to have the balance checked on the 17 inch wheels and I'm not complaining!

Occasionally you can get a slightly wobbly rotor caused by some rust or foreign material between it and the hub.

Check the wheel weights . Does one have a huge amount of weights ?You may have a slightly bent rim. Sometimes they will only wobble at a certain speed.

Well all 4 of my lock bolts and the spare lug bolt I have all weigh exactly 99 grams! At the end of HPDE and driving home last night I started to get a loud drone from the back of my car at highway speeds. I think it might be a wheel bearing or CV joint. I'm going to have to bring it in to be checked out. The check engine light also came on yesterday. My indie garage was also at the HPDE. They checked the code, it was miscellaneous cylinder misfire while cold, they cleared it and it didn't come back in 2 more track sessions or on the way home, but I'll get that checked too. I don't want to drive all the way up to Mt. Tremblant and have an issue on that trip!

One more thing...
 

....sometimes if you did not properly torque down your lug nuts in the proper pattern, and to the proper torque spec, you can get a warping of the rotor hat that can cause vibration.