The overdrive is part of the transmission. I would recommend that you have it diagnosed before replacing the transmission.
A technician can connect a scan tool and see if any codes have been set to tell them what the problem may be.
Sometimes it can be something as simple as changing the fluid.
There are also sensors that can go bad and not allow the vehicle to go into overdrive.
If you do need a new transmission, I would check into getting a re-manufactured one...it will save you some money. Just make sure that you get some kind of warranty on it. Good luck.
〉 Answered on Jan 5th, 2008 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines