Assuming 27" tires, 3.73 axle, .75 OD(5th gear) ratio, then at 85MPH you should be at about 2,950RPMs
If you floor it and it kicks down to 4th gear you would see 3,950rpm at 85MPH
2003 3.0l is rated as 152HP @ 5,200rpm
And 180ftlb torque @ 3,900rpms
So yes you should have a little more "get up and go" even at 85MPH, but torque is what you want and that peaks at 3,900rpm, so above that you only have the HP keeping you going which does add speed but slowly.
You have the 5R44E 5-speed automatic, which is generally a good transmission.
It is controlled by the computer.
Shifting is based on RPM and speed, along with throttle position(gas pedal).
WOT(wide open throttle, foot to the floor) should cause torque converter to lock up.
One thing you can check is if you get WOT using the gas pedal, Rangers are known for stretched throttle cables.
Open the hood, engine off
Have someone push the gas pedal to the floor
Then see if you can open the throttle any more manually
If you can then cable has stretched.
Google: ranger throttle cable mod
Basically you need to put a zapstrap(ziptie) on the end of the cable inside the cab, this shortens the cable so you get full throttle back.