Hi--
As always, I have a little bit different perspective. Know that my background is working with ADHD kids (my degree is in developmental psychology) and both my kiddos are super sensitive, so I do have some knowledge on this topic.
A few things. First, if you haven't already, try taking chemicals out of her diet. Studies have proven over and over again that it is not sugar that makes kids hyper. However, when testing food dyes, preservatives, and MSG, all these things made kids more hyper.
Second, have you tried Rescue Remedy by Bach's Flower essences? You can get it at Whole Foods or Vitamin Cottage or Sprouts. It is kind of like a homeopathic, but it works on the emotions. You can get drops or pastilles (which my kids love). You could also try something called Calm Drops by Historical Remedies. They are homeopathics designed to calm you down. I take them when I get stressed, and they work great for children having tantrums. The only places I've found them are at Vitamin Cottage and the Vitamin Shoppe.
You could try herbs--Wish Garden and Herbs for Kids both make formulas designed to help children stay calm. You don't want to use valerian unless you want her to go to sleep. Again, you can find these at the above mentioned stores. (In fact, I'm pretty sure they are made in Boulder). You could also try magnesium supplementation. There is a magnesium powder, pretty sure it's called "Calm" that you just add to water--my youngest loves it. Be careful though--too much will give her diarrhea.
Check out Stress Free Kids on-line. They have meditation CD's for kids and I started my oldest on them when he was about four. This could help her calm down. You could also try yoga with her---there are DVD's out there especially designed for children and families.
When she gets into tantrum mode and can't calm herself down you could try literally putting her in a "hug" hold. Put her in your lap and cross her elbows one under the other and pull her hands towards her back. Let her struggle against you--as long as you are making sure your hold isn't too tight she'll be fine, but she'll tell you that you are hurting her. Let her take her "rage" out on you, telling her all the time that you love her and you will let her go as soon as she shows you that she can calm down. Hugging contact helps the nervous system calm down. Depending on how upset she is it could take a while for her to calm down. Just calmly rock her and give her love and tell her that you are keeping her safe. The calm drops and rescue remedy will also help you calm down so you can handle her when she gets into this mode.
Finally, try teaching her how to "ground" herself. Most kids respond to either being a flower or a tree and growing roots into the ground. Then teach her to put a bubble of light around her and this is her "safe" zone. No one can touch her when she is in her bubble. Or for my boys I used a clear bouncy ball--anything that came at them would just "bounce" right off. I would guess she is sensitive to the emotions of others around her and to stress of all kinds and her tantrums are her way of releasing that stress. By teaching her the "bubble" she can insulate herself from over-whelming emotions.
On the topics of meds---I would try the natural route first. Most kids have side effects from pharmaceuticals. She may be calmer but she may not be herself anymore either. I've seen kids lose all personality---almost zombie like--but they were calm. I think this would make you sad. Oh, and you could try inositol power. It's related to b-vitmains and helps the brain and nervous system calm down--it also helps with digestion.
As always, let me know if I can help further. Good luck!! I hope your younger daughter is doing better now, too.
J.