1994 Other Ford Models

Transmission refuses to shift by itself at lower speeds. If we gun it it will shift from 1st to 2nd at 35 mph from 2nd to 3rd at 55mph, no slower. I can cause it to shift out of 1st by using the colomn shifter but not into 3rd at lower speeds. Fluids checked out good at cold, added some at hot when checked, problem same. Sudden onset, no warning. If I leave it in drive, it won, t shift out of 1st untill 35mph. Is this the modulator or should I look elswere for my solution.