[Joseph Smith]

First of all, does it only curve in autonomous, or in teleop as well?
If it's only in autonomous, that would indicate a software problem.
If it curves in teleop as well, it could be any number of problems.
Are the speed controllers all calibrated? Are they all blinking at the same rate when it's driving? Are all the motors running in the right directions? If all of those things are good, then it's likely a mechanical issue. I've seen over-tensioned chains, over-tightened wheel shafts, bad spacing in gearboxes, and chains rubbing on parts of the frame all cause problems like this before.
If you can't figure it out, could you compensate by powering one side more than the other in auton?