Jambo, Melanie! You have several options for your expedition from Disney's Animal Kingdom Park to Downtown Disney. Make sure to give yourself plenty of time to reach your Advanced Dining Reservation at
T-REX. The trip to Downtown Disney can take quite a bit of time, due to construction as this area is transformed into Disney Springs.
You are more than welcome to utilize Disney bus transportation to reach all points of interest while on-property. If you do plan to hop aboard the Disney buses, you will need to make a transfer at another stop, as the buses do not travel directly between Downtown Disney and any of the theme parks. Your closest connection could be made at Disney's Animal Kingdom Lodge. From Disney's Animal Kingdom Park, just look for this Resort's section at the bus stop. Once at the Resort hotel, exit the bus and look for the Downtown Disney pick-up area, which should be located nearby.
You did not mention if you are staying at one of the Walt Disney World Resort hotels or at another site off-property. This is going to affect how you will depart Downtown Disney. If you are staying on-property, just head back to the Resort bus stop once your time at Downtown Disney is complete and hop aboard the bus headed to your Resort hotel. If you are staying off-property and have a car available, you may want to use it that day, as you could have difficulty returning to Disney's Animal Kingdom Park to collect your car if the park has closed for the evening. If you are staying off-property, do not have a personal car available and used your hotel's transportation to reach the theme park, then you will need to utilize alternate transportation options to return to your hotel, such as a cab, hire a car service or use Uber.
I hope this helps you plan your day a little easier. If you have any further questions about this, please let me know!
Brandy