2001 Saturn SC1 Transmission Issue

My daughter has a 2001 Saturn SC1 with 100k+ miles that has recently developed an issue I believe is transmission related. When the car shifts gears (up or down) it makes a loud "kerchunk" sound, accompanied by the car shuttering as if it's driven over a large pothole. It almost feels as if the transmission is ready to fall off the car. She was a little low on transmission fluid, which we topped off, but it didn't help. This problem seemed to appear overnight. Any suggestions??? Thanks.