Hi L.,
I'm going to give you the same suggestion I gave someone on here last week. Have you ever tried the contract system? It worked really well for my 5 yr old when she was going through a difficult phase (I have a 5 yr old daughter, 22 month old daughter and a 5 week old son). I use it for my child clients and help the foster parents of my clients set one up at home. I have seen it work well for children who have many behavioral difficulties. In case you are not familiar:
Have a blank at the top of the page for the child to write his/her name (or you write it). Read to the child what you wrote/typed on the top of the page - something like: "I am a great child. I want to behave for my parents. I agree that I will do the things on this page if I do not behave." It should read how he will understand - just make it appropriate for her age and maturity.
I use excel to make a chart but you can write it however you want. The point is to identify specific behaviors that are problematic then have a consequential chore for each behavior. Given her age, I would do two at most. For example, you are trying to stop her saying something negative. Talk to her and teach her what she can do and say when she is angry to appropriately express herself (I know, you are probably laughing at the thought of her appropriately doing anything!) Write/type "Saying I don't like you" under the title, "Behaviors to Improve". Also, in another block under "Behaviors", write/type "Saying I'm going to hit you". I would address both individually because she may stop saying one and continue the other if each one does not have it's own consequence. Next to each one, under the title, "Consequences", will be a chore that is age-appropriate. Try to pick something that she would not like doing. For example, my 5 yr old loves to muck our horse stalls so I can't use that as a consequence like I do for my clients. If you think about it, you can probably think of some type of work that she does not like doing.
The point of this system is that it is a contract that they sign and agree to. You are not the bad person if they must carry out their consequence. Yes, it takes some time, but it works and I have seen it work with 2 yr olds. It is great for teaching self-discipline, self-control, consequences for actions, responsibility, and confidence. Another example, we had a teenage relative of ours live with us for 3 months last year. He came from a home where there were no demands on him so he could do whatever he wanted, but at the same time, he was abused. He was very difficult when I told him to brush his teeth or bathe every night. In an effort to give him some kind of independence but enforce what was expected of him, I created his own contract that hung on the fridge. If he did not brush his teeth on his own, without me reminding him to do it, he had to scrub the tiolets upstairs. The first two days I asked him if he had brushed his teeth and reminded him of the chart before he cleaned the toilets. AFter that, I never said a word. I would hear him curse under his breath when he realized he had not brushed them then I saw him clean the toilets. This was him agreement - I never said or did anything at this point. This took one week then he never forgot to brush his teeth again.
IF the child refuses to abide by what has already been discussed and agreed upon, the child does nothing, loses all privaledges until he/she complies with the contract. It takes patience at first but it will work. I had parents in a parenting class that I taught two months ago who tried this with their two-year-old son and it did work. He was physically acting out against his parents and his older brother.
I hope this helps! Good luck. I hope you find an answer soon.