Big Horn 06

Hey guys, I'm trying to help a friend of mine with a trans issue. He has a '99 Dodge 1500 with the auto trans and he tells me the trans won't upshift until it's essentially at max speed for each gear, revving out. If it was an old car, I would say the modulator but I don't know these newer trans. He tells me he replaced the sensor in the rear end. Any suggestions?