I would do exactly as you stated. The front input seal on the trans when it gets hot will expand and cause a leak. Which once it is cold again it reseals and no more leak. But that seal needs to be replaced.
How ever the torque converter can go bad. But usually when it does one of two things can happen or both will happen.
1. when you put the vehicle into gear no matter what gear, the engine will die because the torque converter is locked up and will not unlock.
2. When the trans is put into gear, no matter what gear, the truck will act like it is in nuetral and not go anywhere.
Does she recall if the od light was flashing. The trans shop should try and pull transmission trouble codes. If the od light was flashing then the trans went into limp mode. Which will cause the trans to shift real hard and basically jerk the truck when it shifts.
3,000 seems awful high for a torque converter I had my trans completely rebuilt and did not cost that much.
For 3,000 you can pretty much rebuild the whole trans with high performance internals from Suncoast, BDS, or other premium places.