I have not done it yet, plan to do it before next camping season and plan to follow these instructions:
http://www.nissanoffroad.net/how_to/transmission_cooler/transmission_cooler.asp

As far as a cooler, they are universal and usually rated according to the gross vehicle weight (including cargo and what you're hauling).

I will probably go with the 1301 from Permacool or comparable size from a competitor:
http://www.perma-cool.com/Catalog/Cat_page05.html