Final answer:
The leak at the rear of the transmission is most likely caused by a failing output shaft seal, as this is where the transmission fluid can escape when the seal is compromised.
Step-by-step explanation:
If a technician notices a leak coming from around the rear of the transmission, the most likely cause of the leak would be the output shaft seal. This seal is responsible for containing the transmission fluid within the transmission as the output shaft exits the transmission to connect with the driveshaft. When the output shaft seal is damaged or worn out, it can lead to transmission fluid leaking at the point where the transmission and driveshaft meet. It's less likely that the input shaft seal, shifter shaft O-ring, or speedometer housing O-ring would cause a leak at the rear of the transmission as these components are located at different positions within or on the transmission.