GEN light...

Ok, a few days ago the GEN light came on, on my 96 XJ, nothing seemed to be wrong after a couple days of driving it, but today i noticed the voltage gauge is around the bottom red zone, but it still cranks like the battery is fine. COuld this be the alternator? my CEL is also on for the downstream O2 sencor, There is also a loud high pitch nosie that sounds like the fuel pump when you turn the key to the on positon, but the noise is loud even when driving. not sure if that has to do anything with the GEN light but it started soon after the light on. Also the GEN light always on, but somtimes turns off. Any ideas?

Why not verify alternator output with a meter? Put a lead on each of the battery connections. Use DC volts range. At high idle, you should have "approx" 14.4 volts. Alternators can also be bench tested by many parts stores for functionality.

As for the noise, it's hard for us to help you with that. Have a buddy help you isolate. If you hear it right when the key is turned to the ON position, it definitely could be the fuel pump as the pump energizes and runs for 2 seconds when the key is turned on. Fuel pumps "sometimes" get loud as they age. If you do trace the noise to the pump, it might be a good idea to replace it. IMHO, increasing noise/whine from a fuel pump oftentimes means the pump is close to failing.

For the check engine light, post the code here for comment. Troubleshooting oxygen sensor codes is not always straight forward and a lot of oxygen sensors are installed which don't always result in the CEL going off ie; root cause can be other things besides the sensor itself.