I agree with Jick. Know first the magnitude of the A/T's defect before replacing it. The A/T has so many parts that are highly dependent on each other. A failure in one part can manifest itself as a multitude of symptoms and problems to which an untrained mechanic would easily deduce that the whole A/T is shot. Some of these parts can still be replaced economically, if you're lucky enough. Like I mentioned a few posts back, it could just be a stuck solenoid or a faulty gear selector switch/wiring.

If you're technical enough and up to the challenge, get your hands on a shop manual (Google it). There are a few tests there that can be done without pulling down the A/T.

I would put the idea of replacing the whole A/T on hold and do so only as a last resort. For all we know, the replacement A/T could very well be in the end of its life already.