Interesting update - This weekend, I drove ~750 miles. During the first 500 or so, there was much rejoicing at the newly found smooth ride. However, towards the end I started to notice a very subtle version of the intermittent vibration I was feeling before. The intervals when it would come in was much greater and the intensity of the vibration was much lower, but still...it was there.
I now have a new theory: The last shop I had the vehicle at said that the struts were getting worn, but they also were hesitant to say it was the cause of my vibration. I think the wearing struts are allowing the tires to bounce and/or wear in such a way that, when the conditions are right, causes the vibration I am feeling. The main problem with this theory is I don't see any sort of cupping or other wear pattern on the tires I just took off, but it may just be that I'm not very good at spotting it and/or it is very subtle. If there is wear (even subtle) combined with worn struts, it may add up to what I am feeling - so back to the cumulative effect of a few things like I was saying in one of my previous comments. If, as moose said, the guys throwing tires are just rushing through their job and aren't experienced enough to know what to look for, there could have been something going on with those old tires that is now being caused on the new tires. So, I'm ordering new struts asap. If this is indeed my problem, I don't want to bork a brand new set of tires.
Looking on rock auto, a set of monroes isn't too outrageous - is that probably the brand to go with?