1990 mercury grand marquis won't shift out of first gear

Have you checked to see if the TV Cable is attached at the throttle body? This cable may become detached because a simple grommet falls apart. If this happens, the transmission has no idea where the throttle is. This is a guess, but a good starting point. If this is not the issue, you have a vacuum issue, vacuum modulator issue, or internal mechanical issue. If the cable has been detached, transmission damage may have occurred already.