This evening while off roading a bit i hit a stump and dented my exhaust pipe after the header and before the cat. The truck seems to make the same power but sings a different tune. What is the cheapest way to fix this ?

 

Welcome to the forum...
If you are looking for the cheapest, its to do nothing
Any sizable dent will restrict flow, its it critical? probably not, but if its changing the exhaust note it might be enough restriction to be concerned about...

If the dent is not likely repairable, you will likely need to have that part cut out and replaced, if its in a straight area you might be able to get away with cutting it out and slipping in a flared piece with some clamps if there is room...
If its in a bend, its replacement only...
Post a picture

 

Yeah, a picture or better a video on how it sounds.