I would say that the 'less is more' approach is the way to go when dealing with any kind of pseudo-sexual behavior in such a young child. What I mean is that dealing with it in a simple, non-judgemental and caring way is generally the best way to address something like this, if at all. The other mama's have given great advice to that effect. Children at this age don't have a sexual desire as we adults think of it so their actions (kissing classmates, showing privates, etc.) is just out of curiosity. I think adults project too many of their attitues and feelings onto children in general and especially in areas of sexuality - just my opinion :)
You mentioned 'Christian' advice - I am not sure exactly what you mean, but there is no where in the Holy Bible that says masturbation is wrong. The Old Testament story of Tamar and the 'spilling of the seed' is often used as Biblical proof that God disapproves of masturbation but that is not the context. God struck down the brother because he did not obey the commandment of kindsman-redeemer to Tamar (I won't go into anymore details here - if you want more, please send me a message). In our family, we believe that God is mostly concerned about the intentions and purity of our hearts and minds and when it comes to this issue it's more a matter of 'where your mind goes'. Because your daughter is so young and is only doing her 'rocking' because of a self-soothing behavior, then there is absolutely no cause for concern as far as 'Christianity' is concerned.
As parents, we can only encourage our children to choose particular things or actions to soothe themselves but ultimately it is up to them to decide what works for them. I would say don't stress about it and chances are she will keep it to herself or give it up altogether at some point. Just give her lots of love and help her feel secure, which I am sure you already do!
Good luck!