I wouldn't rebuild it yet.... Sounds like a component problem... Have a competent trans shop pull the pan and replace the filter... It probably needs done anyway.. Have the tech check to see what is in the pan... That will tell a lot.. Some clutch pack dust and some aluminum flakes are normal... Much debris and the smell of burnt fluid will also tell you if it needs replaced... If all look normal it could be a shift solenoid problem or a corroded connection.. Here is some info I found.. Seems that 2nd gear is the "limp mode" And explains why it does it... Good reading..