For anyone who may find this through a search engine, the issue has been resolved. I believe the part that went bad is known simply as the shift cable. It's a line that runs from the gear shifter to the transmission. The cable became tangled and was making it hard to move the shifter. Unfortunately, the part is only available through GM so I had to take the car to the dealer. It was roughly $100 for the part and roughly $100 for them to install it. For those who want to attempt replacing the cable themselves, the center console (where the shift lever and cup holders are) must be taken apart.