Severe jerking/stalling with 2 emissions codes

I have a 2008 Grand Prix gxp, 5.3L, V8. The check
engine light came on a few weeks ago and
recently, the car began a severe jerk/stall attempt
when initially acceleration and continues until
above 35 mph. It feels as if the fuel is just not
reaching the engine until I reach 35 mph. Also, I
had local repair shop run codes...I got P0137 and
P2270. I was told both are emissions codes and
when I pressed for details, the guy said he checked
and my 0xygen sensor shows low voltage and that
is the P0137 code. Car parts store says not
necessarily true and that it could even be a gasket
failure between the muffler and manifold.
Not sure what to make of it. Also, found leaking on
driveway under the car. No antifreeze, oil or tranny
fluid. It looks and tastes like plain water.
Help!!