Why are the gears harder to shift going up and down on a '96 Ford Taurus?

You may have a clutch problem if the engine and transmission are not being fully disconnected during clutch pedal operation. The clutch master cylinder and slave cylinder are directly related to clutch operation, if these are not working properly it will cause your clutch to slip. If you can locate an inspection cover or hole at the front of the transmission you can actually watch while a helper operates the pedal to see if the clutch is disengaging from the flywheel. You might notice a change in pedal feel this also indicates a problem. The other problem you could be having is an internal transmission problem consisting of damaged or worn shift syncronizers. Syncronizers are placed between every gear inside your trans. they provide the smooth mesh of gears while operating the gear shifter. Your trans will have to be removed and overhauled or replaced if this is the problem