You wont be able to check your throttle cable with the engine apart. Your cable stretches over time, so when you put the pedal to the floor it doesn't actually open the throttle all the way. when you get it all back together get someone to sit in the cab and push down on the pedal, make sure that the throttle body is opening up all the way... Ive seen it many times, there is another thread on here somewhere with a couple of fixes that involve zap strapping the cable on the pedal end. do that for the short term and then get yourself a new cable.

 

And yeah it does sound like either an o2 sensor or a bad cat... if the cat is making backpressure in your exhaust it can lead to cracked heads so deffinatly worth checking it out.

 

cat was just replaced in august. the cat now is doing nothing. its fake.