here is a pic of my 26,000 GVW B&M Cooler
(on the right hand side - that's my license plate)

took about an hour start to finish - and I had to relocate the temp sensor for the rear view mirror - I just moved it over to the drivers side so it wasn't so close to the cooler.

http://www.ricllc.com/javentre/cooler.jpg

if your determining factor is whether the fluid is burnt or not - when you find out cause it's black and burned - you're too late - you've already done damage. Here is a good write up for why you need one.
http://www.gadgetonline.com/Transmission.htm#Transmission%20Cooler: