can't push botton on floor shifter

I have a 1998 GMC Jimmy, 4.3L, 250,000 miles. I can't push in the botton on the floor shifter. I even tried to use the white bypass tab and could not get the shifter to move. I have needed an ignition switch replaced on three other occasions and this does not seem to have the same symptoms. Everything else works, I just can't push the botton. I had to unhook the transmission cable and shift using that. Do you think the shifter itself needs to be replaced? Everything else seems to work, brake lights, headlights, everything else but the shifter button.