The metal hub at the center of the blower fan had become detached from the plastic fan. As a result it was very loud, due to vibration, and it was taking longer than normal to dry the clothes. Eventually the fan not turning properly led to the thermal switch failing as well.
I had to cut the plastic part of the fan away from the metal hub with a utility knife so I could clamp onto the metal hub with a locking pliers. (The metal hub had also become fused to the motor shaft.)
Once I was able to remove the old fan, the rest of the repair was fairly easy. My dryer runs like new now.