Hi,
My 99 V6 had a transmission rebuilt. They did not change the pressure sensors (2) and the one that is responsible for sensing the pressure between 1st and 2nd (I think) was acting up. After they replaced it, the shifting was fine. I know there is another one. It usually did that when the transmission was cold though.

The other thing you can try is to disconnect the battery for 5 minutes and then reconnect. that will reset the transmission controller. Usually when you have a transmissions issues, the D4 light will indicate something but it did not do it with the bad sensor.

Give the disconnect a try but before you do, make sure you have the radio code.