Although the bottom of the radiator acts as a tranny cooler, my A/T light has come on exactly 3 times in the past 10 years. The first was while coming back from Arizona, carrying a full truck load of people and gear while doing 70mph in 100 degF heat. The second time was while bombing around Pismo Dunes at 30-40mph in the soft sand. The third time was while going up an incredibley steep grade for about 1 mile while completely loaded down with people and gear. I figured all that heat probably wasn't doing the tranny any good so I added an auxiliary tranny cooler.
There are 2 types of coolers out there, the "tube and fin" style and the "plate" style. From my research on the internet I found that the plate style is the most efficient. If one is an electrician or an EE, a good analogy for the two types is the tube and fin is series and the plate style is parallel (not to be confused with how one installs the cooler in relation to the radiator).
The TRD cooler is a stacked plate style and costs a small fortune. I opted for a Hayden plate style cooler from the local auto parts store for $50. The quality is just as good as the TRD and costs a ton less. I got the smallest one they had because at the time I didn't know where I was going to mount it. It's a good thing I did though, because it fit perfectly.
Installation was pretty straight forward. Normally, the tranny cooler is mounted either in front of the radiator or behind it, but I couldn't see anywhere it would fit and I was too lazy to take off the bumper. So I got some 1/8"x1" flat bar, drilled some holes and bolted it to the tranny cooler. See below for a picture of this mount.
Then, I dropped the skidplate and bolted the tranny cooler up to some existing holes in the front cross member. Plumbing the cooler was really easy. I took off the line where the fluid exits the radiator and connected the new line. This new line was then routed to the tranny cooler, trying to keep the line as straight as possible, and cut. Another new line was connected to the other side of the tranny cooler and routed to the tranny inlet hard line. Very little tranny fluid came out during the whole process. Previously, I had done a tranny flush and so I remembered which line was the inlet and which was the outlet. Unfortunately, now that I write this I forgot which is which.
To increase the air flow to the tranny cooler I drilled (2) 2inch holes in the stock skidplate. The skidplate bolted right back up without interference from the cooler. I got a chance to test it out on that same steep grade and it passed with flying colors, no tranny light.
Keep in mind that because of the location of the cooler it can be vulnerable to impacts or other damage. If the cooler were to break open and loose all the fluid, you'd likely need a new transmission. But so far I don't have any dents that high in the skid plate.
Update: I did finally get the tranny light to come back on but it was under extreme conditions. I was driving in soft gravel in 4hi going fairly slow, about 15-20mph so very little air was flowing through the cooler.
Here's a couple more pictures:
Update 3-20-07
Since doing a solid axle swap, I moved the tranny cooler up higher and out of harms way.