i fell victim to the precat manifold. my 97 had the infamous crack along the collector and the rest of the exhaust piping was falling apart, some had fallen off completely. so I grabbed the complete exhaust off of a wrecked 01 xj thinking I was doing good. swapped everything from manifold to tip over to mine. ...apparently not having done any research was a poor decision. I learned this after getting it swapped and starting it up. the new style exhaust ports are smaller. so much smaller in fact they will not cover the ports on the head of my 97 xj. so I had a friend tig weld my old one and I reinstalled it along with the crumpled downpipe. I'll have an exhaust shop mate the 2 at the cat. simply wanted to share and maybe save someone else the trouble.

97 left 01 right

 

The 01 you took off is the California spec. Mine (surprisingly ) is the "50 states spec" or whatever you call it. It doesn't have the cats on it like the one you have does.

I can't say for sure, without knowing the measurements of your 01 manifold, but mine looks bigger than that. Complete and total guess, but California is usually stupid about everything else (COFFEE GIVES YOU CANCER!?) so it wouldn't surprise me if they wanted some smaller exhaust ports on there too to limit emissions. I'd be curious what those smarter than me know.