1994 3.0 automatic 158,000 miles. Driving on the freeway 70-75 mph and it won't shift at 4,000rpm. I just got an oil change, was told to try changing tranny fluid? Any ideas? OD going out?

 

Auto should be a 4 speed?
You should check the AT fluid level, the shifter linkage could be out of adjustment or it might be the torque converter not engaging the lock-up, which sometimes feels like a 5th gear change. If none of these it could be the computer not getting a signal to shift into overdrive or transmission troubles.

 

Mine runs just over 3000rpm at 70mph and it is only a 4speed auto like sechott said.