swap your front wheels for your back ones or.............
get all 4 wheels balanced not just the fronts
also get them balanced correctly (weights on both edges of the rim) weights tucked in and hidden behind the spokes, if this can't be done get them to slip the tyre round a bit and then the weights will be able to be hidden, this is dynamic balancing
(static balancing) is the lazy way to balance a wheel without visible weights is to only but them on the inside rim of the wheel, this way is not so accurate
read here about static and Dynamic imbalance
http://customwheelsmarket.com/tirwheelbal.html