2001 Ram QuadCab 4x4 5.9L GAS

My check engine light came on so I bought a new scan tool and the tool is saying Code# PO138 and in my book is says:
(o2 sensor circuit High Voltage) Bank1 Sensor 2
Could someone please help me with this
If I have to replace this sensor where would it be located?
Also My Emergency Brake light comes on and my ABS light comes on(not all the time but when they come on they stay on until I shut the truck off. When my ABS light and Emergency brake light comes on and I'm going around example: 100km/h the speedometer needle goes up and down like crazy between 65-110 when I'm traveling at 100 km/h.
Someone please help with these problems (o:

I'm not sure which one it would be, but the o2 sensors are in the exhaust system. generally there is one before the cat, and one after, but I've heard of some trucks having 4 o2 sensors.
the "abs" and "break" lights sounds like the rear axle speed sensor. it's located on top of the rear dif. when my speed sensor went out, I had the same lights (abs, and break), but my spedo quit working below 20 MPH. sense your problem comes and goes, the sensor may not be bad, it may just need to be taken out and cleaned. however, it it has a lot of stuff on it, it would also be a good idea to check the dif fluid, and gears just to make sure that they are ok.