I first got a Mag Hytech deep dish trans cooler. Normal driving lowered my trans temp from 160-170 to around 140, under normal operating conditions. Also run the diff cover. GREAT products. One of the first mods I suggest on most trucks.
After that, I prefer a trans cooler with a AUX e-fan. Reason is, I tow in some rural spots, and often not traveling at highway speeds, and going up STEEP terrain. This heats it up quick, even with a standard cooler. A regular cooler needs fresh, cool air passing through to be effective at all. With an AUX e-fan, it will pull that fresh air through for you. Great mod, and wont have to worry about temps again !
I like this one:
http://www.troyerperformance.com/cgi...3BTransmission
I believe B&M also offer a nice kit just like that, if you'd rather go that route.
*** just checked it out, looks like ya might save some money going with B&M, or another company such as "Be Cool" or "Permacool". Might want to look around.