I just wrote an entire paragraph and then deleted it before I posted it. I know, I am a moron..
Instead of re-typing everything I am just going to say this. Without knowing anything else about the truck (year, engine and trans, mileage, recent work, other driveability issues etc.) let alone being next to it, it is very hard to diagnose an issue. Especially without knowing any other codes besides this one.
With that said, I think your problem is PROBABLY going to be in your 02 sensors. A rich condition is usually only caused by 02 sensors or fuel injectors.
Now, do not go out and spent 300 bucks on 02 sensors just because i said so

Check your air filiter, make sure its good to go and not dirty. ALSO CHECK YOUR INTAKE TUBBING AND THROTTLE BODY. Make sure nothing is in there blocking things up.
Next, go get your truck scanned. Have it hooked up to a scan tool, and see if and which 02 sensors are giving false readings.
Let us know what you find and we can help you better. Also, double check there are no other codes. A rich/lean code by itself with nothing else is pretty rare. Again, have your truck scanned for current and STORED codes, and also the 02 sensor readings at idle, cruise, and WOT and get back to us.
-Jason