Took it to the local tire shop today. Wheels were all out of balance to some degree. All balanced out, but the two front ones didn't seem to be 100% true (brand new reproductions), so he said that I might still have some slight vibration. Sure enough, the vibration is still there, but significantly reduced. Need to hook up the speedo to see where it kicks in at, but I think it's probably around 70 mph.

Thanks for all the input!

Scott