The best thing to do is to tear apart the lower unit. There is a seal for the drive shaft, prop shaft, an o ring on the bearing carrier at the end of the prop shaft. Depending on the year of motor there should be a seal on the shift lever that comes out of the unit. Don,t forget about the top and bottom plug seals on the side of the unit. Take it off the motor and drain the oil. Use a pressure tester and fill it to about 20psi. Use soap and water on all seals to find bad one. The best thing to do is get a seal kit and replace all of them (not real expensive). When your done it should hold 20 psi for 10 to 15 minutes. It is also good to do a vacuum test on the unit also.
Sounds like an oil seal leak that would cause this. Call up Bass Pro Shops and see what will be a quick fix. Good luck:)