[Only Toyota for me]

If you have the 4.7 v-8, you already have a tranny cooler. It will be in front of the radiator & is about 8"x12" or so. It looks like a small radiator.

If you have the v-6 however, i would HIGHLY recomend you put a cooler on it. Otherwise plan on having transmission issues down the road.
I owned a 99 runner & towed our toyota epic 22 boat for one summer, and the fluid started to turn brown in color. I immediately put a tranny cooler on it & the fluid held up just fine. The bad thing about your transmission is that it is very difficult to check the fluid. The 99 had a dipstick so it was easy.