4x4,
Tranny cooler is an automatic transmission cooler. It keeps the automatic tranny temperature cool. It is needed if you plan to tow often or off-road often...actually, it is mainly for towing...i am not sure about off-roading. It is usually mounted in front of the radiator, which means it is located in front, just behind your bumper.
I don't tow anything. However, i do off-road. Some people feel that if you idle your car often, then it is a wise idea to get a tranny cooler. In addition, there is no harm in having it. Most people will agree that even if you do not tow, the tranny cooler will help prolong your transmission life.
You can either use TRD cooler or Hayden cooler found at your local auto store. I believe TRD cooler is made by Hayden. However, i chose TRD because the mounting brackets and installation instructions are made specifically for the 4Runner. You pay for simplicity!
With the Hayden, you need to fabricate your own brackets. However, it is cheap to buy. The Hayden cooler should cost around $50-70, as compared to $190 for the TRD.
In conclusion, if you are good at installing things, then get the Hayden cooler. However, if you have little imagination (like me), then spend $$ to get the TRD cooler.
I HIGHLY recommend the tranny cooler!