To share a possibly similar accident story. My daughter drives an '07 Acura RDX. She got rear-ended twice in a single accident as she was the first car in the line & successive cars rear-ended each other. It happened exactly a year ago this month so it's still fresh in my mind.

Her car seemingly appeared okay other than a banged up bumper. She drove to her apartment after the accident....no problems. Later in the day drove to campus for a class, went over a speed bump and the driveshaft fell out of the cradle onto the road & the transfer case spilled all it's fluid. Imagine if she had parked the car for the rest of the day & then headed out on the interstate what could have happened.

I know yours is the tranny, but oh yeah it was definitely the accident and the other drivers insurance company owes you a repair job. Fortunately, I didn't have to argue too much with the adjuster, he could see the cause and effect. Good luck.

That's quite a leap without even looking at the car.

The car will need to be looked at to determine what is happening. Did the tail pipe get crushed? What does she consider a slipping trans to be? ( High engine RPM relative to road speed , no power, engine won't rev ? )

Check the shifter bushings as they may have been falling apart already.

The rear subframe would have to be pushed very far forward to out run the slip joint and crush the trans. ( as in rear wheels rubbing front of wheel wells. )