4L60E / 700R4 Forward Drum Inspection; 3rd Gear Slipping and Failure 

This video discusses common problems associated w/the 4L60e and 700R4 forward drum and how to inspect the drum to confirm/deny it's serviceable.
While not mentioned in the video it's also important to carefully look over the drum casting itself for cracks, both inside and outside once it's been emptied of its contents.