Rich burn on 91

I got a check engine light and pulled the code 173.... heated O2 sensor rich burn right side. What causes a rich burn? That is too much fuel, right? How do I fix it? I have a 91 with 4.0 automatic w/4 wheel drive, it has about 103,000 on it. Also I get a vibration in what seems like the right rear sometimes. It doesn't come with any regularity. but does seem to happen at speeds of around 45-55, but not all the time. Some times not for weeks. All wheels have weights on them and all lug nuts are
there, any idea's?

thanks