Ah, that's a hard age, IMO. Old enough that redirecting doesn't seem to do much, but still too young to have words or other ways of expressing frustration.
If you aren't already, I'd give her a little warning before taking a toy away. I don't know about your daughter, but my son has trouble with transitions in general and that's when he gets most frustrated. Giving a 2 minute warning, then a 1 minute warning, then actually doing it seems to help (although not completely).
When she does get frustrated, start giving her some words, like saying, "I know you are sad (or mad) to lose the toy. We'll play again later."
Other than that, I'd try the best I could to prevent it (grabbing hands before she hits), telling her hitting is not nice, it hurts Mommy, and modeling good gentle touches.
It's the repetition that will finally sink in, although it's not immediate. Hang in there, it does get better.