Maybe her "truths" and yours are just different truths, rather than her telling untruths about you and your wife. Surely truth is a matter of perception?
To be honest, I think its always difficult with grandparents as they will do things that the parents don't like, such as spoil their grandchildren rotten, which they couldn't do with their own kids since they were responsible for bringing them up to be grounded, rounded and responsible pillars of the community.
You say that your father and mother in law don't hit your child the way they used to hit her mother - is this really how you would want them to treat her? Spoiling your daughter and letting her have her own way would NOT be good for her on a daily basis, but grandparents are there for the good times, the presents, the biscuits and the cuddles. They aren't there to chastise, punish and discipline because they have already done the hard work!
Good luck and remember that although you may not see eye to eye with your inlaws, and although they may drive you demented, they play an important role in your child's life and, ask yourself honestly, how many times have you asked them to babysit when it's been convenient for you?