What size is the tip you are running ? and is 2 1/4" your stock size piping ?..

I would go with the advice above, timing lash and leaks, and if thats not it maybe it is the cat. There are alot of things that could contrubute to this. I know in my old LS it would backfire with a stock cat, and 2.5" piping, But I replaced the whole system back to a stock style and it got rid of it, pretty much confirming it was an air flow thing. Does your tip have a silencer ? If so, try pluggin that in and listen for it..

Ohh, when you shift, what rpm range does it do this in, or is it all ranges ?