My driver side header gasket bit the dust, also seeming to correspond with this event is big loss of power; my 4.0 jeep cherokee could spank the stang right now, can exhaust leaks such as this cause power loss?
if you have aftermarket headers replace the gaskets with mr. gasket part number #5928 (these have the oval ports) along with a set of locking bolts and you will do away with this problem...
Your O2's will read the mixture running to lean due to more air getting in through exhuast and cuase the car to run too rich more fuel less spark less MPG.