Are you sure that it's your tranny that's leaked up there? It could very well be a bad head gasket leaking engine oil down the side of the block right there. I currently have a bad head gasket doing that.

And how long has this been happening? Sounds like low tranny fluid to me. There is a dipstick with a yellow handle back there. It will tell you how to check the fluid in your owners manual. Assuming you have one.