On my 01 4.0L.......last cat to coupler is 2.5, after coupler is 2.0, after muff is 2.0, tip is enlarged to 2.25. I think i've seen 04+ with the tip enlarged to 2.5. Don't know for surethough. That's on a 4.0L, remember.
You could just go to a junkyard or check out craigslist and hack off one of those couplers to the muffler, and use that as a starting point. Should be 2" is what's needed.