If it does turn out to be one of the forward gears, you have go replace/rebuild the entire thing. It's fairly new, so it could be a computer issue. An over full transmission is just as bad, if not worse than one that is slightly low. You always check the trans fluid with the car running and preferably in neutral. You can put the e-brake on, or just back up to a curb or something and let it sit in neutral. Oil on the other hand, you check with a cold engine and the car not running.

Vehicles are money pits. Unfortunately.