vehicle will not start in park but will start in neutral, what could be the issue?

Best I can remember (it's been awhile since i worked on a 91) .. Park neutral switch is located just above the transmission shift linkage on drivers side of the trans .. It is 'threaded' into the transmission case .. Try this > Force shift lever further into park (push or pull upwards) with one hand while turning key to start .. Does the engine crank now?