the only reason i wouldnt think its the cat is that it is brand new. i had to redo my whole exhaust system when i registered the truck 4 months ago. DAMN CALI SMOG LAWS!!! and it doesnt smell of gas any more that was taken care of. it was just a hole in the exhaust pipe where there was a crappy weld that broke. yet it does hesitate on acceleration, plus when cold it holds 1st gear longer than it does warm.
Have you tried to pull the codes stored in your ECM by attaching a jumper between the ground (pin A) and diagnostic terminal (pin B) on your ALDL? You don't need to have a code reader to pull the codes but, it makes it easier. HTH