Disney is great - if you need help, they'll do what they can to give it to you!
You don't need to provide any proof of disability, doctor's note or benefits' information; you just need to explain your needs and how they can help.
As CK said, an
ECV or wheelchair might help you, by providing a place to sit when needed. You can rent these in the parks or from off-site companies, though do be aware that the parks often run out of
ECVs. The parks are very accessible, and most lines you can go through in the wheelchair as you would on foot. You don't need to use the wheelchair all the time, and you can park it in the stroller parking when you need to stand / walk.
You may want to avoid, or at least think carefully before riding, anything with a red triangle warning. These rides pose risks to people with various health issues, and could contain sharp turns or jolts that could further harm your back. If you're unsure about any of them, you can ask the CM at the entrance to the line, and they can give you further details.
If you're staying at a hotel with a hot tub, I suggest you try to use it at the end of each day, as it will relax your muscles before you sleep. I've got back issues due to Fibro, and I felt a lot better on days after doing this, than the days when I didn't. If you can't get to a hot-tub, a hot bath would help too, as would a few gentle stretches.
If you need to lie down whilst in the parks, the first aid centres (marked on the maps) all have beds that you can use if you need. Just ask them if you can use a bed, because you have back issues and need to lie down, and they'll show you to a bed.
Are there any particular things that you are worried you might have problems with? We might be able to help you find solutions, to make your trip more enjoyable.
Good luck and have a great trip!
