2004 Dodge Caravan Car doesn't want to change gears

I have a 2004 Caravan that had a rebuilt transmission put in about 1 1/2 years a go. It recently started acting up. When I get to about 55 miles per hour it feels like it doesn't want to go into the next gear. If it does change gears I have to run about 75 mph to keep it running smooth. ( If I had to guess it is not wanting to change from 3rd to 4th gear)

It has just been serviced. All fluids and air filters changes.

What does this sound like to you?

Thank you in advance for your time