Good for you! Good for you for seeing help and good for you for continuing the dialogue until you have gotten some good suggestions that will help you. Don't feel bad about this. You need some answers and this is a fairly difficult topic to discuss. As a side note, I am sorry that you are going through this. I can imagine how embarassing and frustrating this must be for you.
I haven't read all of the responses but I am wondering if the person that suggested the booster seat meant the booster seat for the kitchen table? Taking her out of the high chair and putting her in a plastic seat with no buckle?
Also, does your daughter seem to require extra stimulation with anything else? Some kids just need extra stimulation. Does she walk on her tip toes, bang her head or any other behavior that stands out as needing extra stimulation? If so, maybe this is a bigger issue to investigate?
I hate to bring this up but I must also bring up the idea that maybe she was in a situation where this was or is introduced as appropriate behavior? Is she in childcare? Has she ever been around other kids that might have or be doing this? As a Mom, this is something that I would consider too. It's ugly but certainly something to think about.
Definitely check for yeast infections. Also UTI? Obviously itching and burning sensations would cause a child to explore relief, especially a child who can't communicate these issues.
If it's a "feels good" situation, do you think that she is old enough to have the conversation with her that it's not appropriate behavior in public but something she can do in her room? This too, obviously, is a controversial way of handling things but it's certainly how we handled our son when he would play with himself.
Anyway, I hope that you find your answers and that one of us "Moms" can help. :)
Good luck.
N.