I used a Hayden 32,000 GVWR unit. Biggest one I could find and would fit in there. It was reported to me by another member that Hayden makes the ones for Ford. It was $125. It was very very easy to install. I went to the local transmission place, gave them my keys and sat and read magazines for about an hour!
(actually the cooler was about $65 or $70 or so of that and I had it done because I am a chicken when it comes to cutting perfectly good tubes and things)
Oh... another thing I learned: You will make a choice as to wether to leave the internal cooler (on built inside the radiator)in line or to by pass it altogether. On a newer truck, with a radiator in good shape, well maintained and with little chance for the internal one to leak across to the engine coolant, I chose to leave it inline for additonal cooling. If you do so as well then the fluid will run through that one first and then into your new auxilliary one and then back to the tranny. Got it?
BTW: This was in my 97 F150 4X4 lariat. I have the factory system in the SD.