I wouldn't worry, but I would be concerned. First, sit down with her and ask her to repeat the sounds of the alphabet. Write down which sounds she cannot pronounce or say clearly. Make a list of words she can say. Next, I would begin right now encouraging her to speak. When you know she wants juice for example, say, "YOu want juice?" Then get to her level and say, "Mommy wants you to say, I want juice." See if she can say that. IF you know she can say it but is refusing because of that stubborn streak, then tell her she must say she wants juice and once she says it, you will be happy to give it to her. You need to begin encouraging her to speak when she wants something. I know this will be a challenge for you . I've experienced this with my daughter. She knew that I understood her so I would automatically give her what she wanted even if she said "juice" instead of "I want juice." Pretend you DO NOT understand her and have her believe that in order to be understood, she must tell you and help her by saying a sentence and having her repeat it. Say things like, "You have to go potty?" You need to tell mommy, "I have to go potty." Can you repeat that for me so I understand that is what you want?" Don't allow her to use gestures like nodding of her head or pointing to get what she wants.
Also, do you have an Easter Seals in your area? They do offer speech therapy and it might benefit you to have her speech evaluated. What does the pediatrician say? There may be county sponsored organizations to help with the cost. I would begin with making a list that way you have something to work with and you can show your ped what she can say and not say. Once she is in school, your school should be able to provide speech therapy if she needs it.