UFgatorgirl,
I agree with what the others have already posted. You have to go through Rush for yourself -- not for anyone else. And your friend needs to respect that.
Trust me when I say that no one should ever "assume" that they are going to join the same organization that their other high school friends do. The way Rush works, you just can't say that -- because you have no control over it. Your friend needs to realize that there is a big possibility that you and she will not wind up in the same sorority at UF. Rush there is very competitive and there's no way to tell where either of you may end up. This isn't to say that you and your friend shouldn't go through the process, though!
It's a great way to meet people and get acclimated to college life!
I'm sure that your friend is a little apprehensive about going to a new school and meeting a ton of new faces. That's probably why she's wanting to stick so close to you. But if y'all stick together then you won't ever be able to open yourselves up completely to meeting new friends. Y'all may get lucky and both get bids to the same house that you both love. But you may not. It's not your call to make -- and your friend should not be making you feel guilty (especially so far in advance) if it doesn't work out.
Another thing: Being in different sororities won't change your friendship if you don't want it to. There are many opportunities for members of different GLO's to get together and socialize, and there will always be some free time for you and your friend to get together one-on-one. So, please don't feel guilty or upset by the way your friend is acting. She's being a little inconsiderate, and that isn't your fault.
Good luck with Rush and with your friend. I'm sure things will work out!