Begs the question pray tell where in the hell would you be doing 70 anywhere BUT on a highway? cwm3... speeds over 70 but only does this on highways.
Seriously, that's the speed to feel a thrown weight. Have them ALL rebalanced just to eliminate a big variable. Don't fret about the cost. You need to be able to diagnose if a ds/u-jt./tran mount/alignment problem is coming on.