93 dodge dakota not shifting correctly

I have a 93 dodge dakota truck 3.9L, it's automatic. When I turn the truck on and put in gear I press the gas pedal and it takes off slowly then speeds up. When going down the highway with the gas pedal pushed down, I get high rpms but the truck does not go to the next gear unless I take my foot off the pedal. Could it be the passing gear? Please advise. Thanks