I'd like to start out by saying that speech has never been a problem for my daughter, Kayden. (It has been all the other areas that have been issues)... she doesn't have her "L's" or her "TH's" yet which is rather typical. Anyway, here is the strange thing she has been doing for the past 2 months or so:
Kayden is substituting letters in her speech. She'll do it inconsistently, but more so when she is excited or "in motion".