Update. I hovered and hounded at the tire place, and made them redo the rears, too, which they hadn't wanted to. The rebalance seemed to do the trick, as I didn't note any problem driving to work this morning, with a brief burst up to 75 or so. The Kumhos are sure grippier than the old MXV4s, though probably not a fair comparison at this point. Quiet and smooth riding too, and much better directional precision.
One question - when originally putting on the new tires, the store lost one of the plastic trim caps on a lugnut. Are these available from any source?