I have a 1995 K3500 with a 7.4l and 5spd. The truck idles very high in nuetral (1400-2000rpms). When it's in gear, it drives fine. But once it goes in nuetral, the rpms rev up. What could be the problem?

I just replaced the O2 sensor. The check engine light is still on. I did not check the codes because I'm having a hard time finding a place with a obd1 scanner. Any suggestions would be great