Yes, in Marie's case it was the torque converter seal. Having said that, I'm told it's also common (?) for these boxes to leak through the bolt holes where the bell housing bolts to the oil pump.
In addition, the problems with Marie's box were not limited to the seal but because the bush into which the torque converter nose locates had worn, allowing the torque converter to run out of true, overcoming the oil seal, which wasn't in bad condition. In the end the bush had to be replaced by a specialist and a refurbished torque converter obtained.
So, I would say get hold of a spare seal and drop the gearbox out (always remove the torque converter and gearbox together to avoid damaging that bush..) but, if there's any scoring or wear evident on the nose of the torque converter and if it's not a tight fit into the gearbox with little lateral play, some additional work is likely to be required. In addition, have a look around the bolts securing the bell housing to the box just in case one of these is the source of the leak.
The seal is just like a conventional oil seal but with 3 screws through a little metal surround to hold it in place. No problem to pull the old one out and drift a new one back in.
Kevin