who can modify computer?

I have a 97 caravan with a 3.8 v6. I would like to have the speed my converter locks up from approx. 44-45 mph, changed to 36-37 mph. I live in florida and most of the city speed limits are 35 and 40 mph. I think my gas mileage would improve if the torque converter locked up earlier. Does anyone know who modifies this model. I see modchips for all different diesel apps, but not my van. chuckq@tampabay.rr.com ..................ICU cq2