I am pretty sure you need 2.5", I think they changed from 2.25 to 2.5 in 2000.
As for the ebay muffler, looks to be a real Flowmaster. So if its what you want and has the right inlet\outlet offset it looks like a decent price. That's about what I paid at a local parts store for the last flowmaster muffler I bought.
Another option with a similar sound is the Dynamax ultra flow, I have one on my Silverado and everyone thinks its a Flowmaster. Its nice and loud with a good tone, sounds a lot like a flowmaster. If you are looking for loud I would recommend it, haven't heard it on a 4.0 but the 4.8 v8 in my truck sounds great and gets a lot of complements.
"Time you enjoy wasting, was not wasted"- John Lennon
"The Jeep is a manly device. And a womanly device. An American device. It’s a fitting instrument to transport the free people of a free nation with the respect to which we are entitled and the dignity that we deserve. Okay, we’re a little crazy to have a Jeep for a daily driver. But if we go off our meds, we might wind up in a Prius"-P.J.O'rourke