Anand,

Yes, post tib can be an issue here.  Also calf suppleness in general.  Smash those areas out and also look into the episodes that deal with ankle mobility.  For me, the heel ball whack, freeing up the calcaneous, and band distracted ankle mobs help me keep my feet firmly on the ground while squatting.  I broke my left ankle playing football 15 years ago and it definitely has less mobility then my right.  I will do the band distracted ankle mobs between squat sets even.

Good luck.