Maybe bushes need replacing?
I have got the same issue and have been told that my bushes are worn(I too had wheels balanced and tracked a month back)
All my wheels are balanced, but if the backs are on the front I get a wobble. Took ages of faffing and repeated balancing to realise this... None of the wheels look buckled, all are true. Really weird. What tyres you got? My prob only became noticeable when I fitted goodyear eagles... (EXCELLENT TYRE BTW)