I would use positive reinforcement. Take a yardstick or riding crop, whatever is long enough to reach behind, and wrap a sock or cloth or small piece of sheepskin around it and duct tape it on so when you touch her with it, it will have a soft and pleasant feel to it. By using this tool you will be able to touch her in the sensitive spots without being in danger of being hurt and you will have more confidence and patience in teaching her to accept being touched, anywhere.
Then, with her in halter and lead just start rubbing her with it in the places she accepts contact already. Once she is relaxed with that, start moving it ever so slightly back and rub, rub, rub. When she allows you to move even a part of an inch back, stop and give her a good 30 seconds or so of scritchies in her favorite spots. Then repeat starting at a safe place and try to work your way back another 1/4 to 1/2 inch again. If she kicks or bucks that means she is just not feeling safe with it that far back, so just totally ignore it and start rubbing her in the safe places again. Just keep doing this and try to work on it for 3 to 5 minutes each time you handle her. Once you get the feel for it and she understands what is happening you will be surprised at how quickly you progress. She will learn to trust you and know that you are not going to push her into something she doesn't understand.
Like a previous poster said, we need to teach our horses that they will be safe when we touch them ANYWHERE. It is their instinct to protect those vulnerable areas and it is our responsibility to help them to understand that we are not a danger to them.
Your goal should be to eventually get her to go "HO, HUM" when you touch her anywhere, between the hind legs, her udder, her belly, her rectum (in case you need to take her temperature), all around and under her tail, and of course ALL parts of her head. Once she is HO, HUM with the stick, then repeat the whole process with your hands. It takes just a few minutes a day but it will save you SO much messing around and save her any anxiety in the many years to come when doing daily tasks.
I have used this method several times with horses that do not like being touched somewhere and have had excellent results with it. I feel it helps with creating a trusting bond with the horse and I wouldn't do it any other way.