Hi there, Amber!
Disney Dining reservations are definitely a key part of the vacation experience. They can be made either
online or by calling (407) WDW-DINE.
You are more than welcome to walk up to any of the restaurants on Disney property - including
Play & Dine at Hollywood and Vine and at
T-Rex - and ask for availability. In more cases than not, the Cast Members at these eateries will do their best to help walk-up Guests with seating; you will find that cancellations and no-shows are daily occurrences at Disney World, which might help your situation.
I'd also suggest calling daily (twice, if you can -- morning and evening) each day until the day of your trip. You never know what possibilities will open up. Feel free to call T-Rex directly, too, at (407) 828-TREX (8739).
Good luck!
See you later, celebrator,
-Deborah B.