What would make a 94 Aerostar with the 4.0 only shift into overdrive at 45?

That Is The Way Overdrive on your vehicle is designed to operate. Consider Yourself Lucky, You Are Not Overloading The Transmission By It Not Shifting Into Overdrive Until 45 MPH If It Shifted Into OD At A Lower Speed Say 25 to 30 MPH It Would Put Too Much Load On The Transmission That Most Likely It Would Fail Before Long.