I have a 88 2.3 liter 4 banger that won't shift into overdrive. Shifts great 1--2---3 but not into overdrive. Can this be fixed without getting a whole new transmission?

 

Yes. get the OD band replaced.

 

Make sure the bands are adjusted properly and the fluid is clean and full. Also, there's likely a switch in the car that prevents OD from engaging at WOT. Make sure this switch is functioning properly (could be a mechanically activated switch, but many are computer controlled through the TPS).