My now 5 year old ds was diagnosed with SID about 2 and 1/2 years ago.
First, with the proper help and a good therapist, GREAT GREAT GREAT strides can and will be made!! Just take a deep breath, *maybe buy some ear plugs*, and grit your teeth.
My son was a walking melt down for about 5 months when he was about 2 and 1/2 years old. If we would go into a resturant, he would SCREECH at the top of his lungs, crawl under the table and scream some more. Even a trip to the park just found him standing there crying instead of playing on his favorite equipment.
However, all of those problems aside, Disney has always been a positive thing with him. He is very into Disney movies. Seeing the characters he is comfortable with help him react well at WDW. I order the free planning videos and have asked other DISers for photos of resort, the ships, attractions, shows, etc. We spend hours going over photos together, listening to sound tracks, watching the planning video and talking about what will happen at WDW. I also show him the pictures at allearsnet and here at the DIS boards to get him used to what we will be seeing, where we will be staying and what we will be doing. Also, we have used his stroller as a "safe spot" for him (suggested by his therapist, to make vacation easier, give him a safety zone that you can take with him so that he always has a way to control the out of control sights, sounds, smells, etc coming at him way to fast for him to process). We put him in the stroller, give him his favorite Crush stuffed animal and a cold drink (he is easily refocused on cold things now, he wasn't always but now he is), face him towards one of us and try to get him into his own zone.
I think the most important part of preparing for him to have a good vacation was just going over and over and over photos and videos. Some of the Disney Sing Along Song videos feature the parks and water parks and really helped him as well.
After just over two years with a therapist he is doing so well you don't know he has issues close to 90% of the time. He is really making progress in leaps and bounds. It was hard to see the progress at first. It felt like months of pounding my head against the wall (watching him do basically the same thing) with no real progress to show for it. But looking back, the strides were being made even then.
I hope some of these suggestions work. Hang in there. Your dd will improve. She is very lucky to have a mom like you who is willing to see she has a real problem and get her some real help...and you are very lucky to have such a speical little one!!
Enjoy your vacation!!