Lots of threads about placement of temp. sending units. A number of opinions too. My '98 had the sensor in the pan. My '06 uses a device attached to the outside of the output cooling line. I prefer the latter since it gives me faster info about what's happening. It also doesn't need to open up the transmission in any way for installation. Geno's sell similar devices for about $32.
IMO, it's the quickest, cheapest, and best solution.
Dan