94 Corsica, high rpm shift

I have a 94 corsica, 2.1, 4 cyl., 3 speed. The transmission doesnt seem to want to shift up into the next gear until the rpms get really high. Otherwise she runs great. I changed the fluid and filter and the pan looked great. A little grey dust, no shavings. This did not help the problem at all. I have tried driving and hit 45 mph in 3rd gear, then shift to neutral and back to D and the transmission does not shift up. I am not a pro mechanic so I need new ideas and rejection or confirmation of my own. Im thinking vehicle speed sensor or engine speed sensor? Do one of those report when to shift? Or do i need to just replace the shift solenoid?