I know when I did my gear change from 2.78 to 3.73, my th400 shifted into third really fast after second gear, and the engine was screaming (close to 3000 RPM) at about 50 MPH. I take it, that it was shifting fine prior to the gear change and you have checked the fluid level? Yea, make sure your vacuum line is hooked up to the carb (ported) and no vac leaks and if good check the modulator.