1998 XJ, 4.0, Auto, 4x4

I'm trying the vacuum test recommended in the Haynes manual to see if I have an obstruction in my exhaust system, and it's confusing the hell out of me. I hooked my vacuum to the port on the intake, hit the throttle and watched the needle. It drops way down, to like 5hg, then shoots up to 25hg and then goes back to 18hg. It idles at 18hg. So, I don't know what those numbers mean. Does it sound like the exhaust is obstructed? I'm suspecting the catalytic converter because I hear something rattling under the rear passenger side floor board. I checked all of the heat shields and it's not that, but it's progressively getting worse, and I want to be sure before I just start throwing parts at it.

 

Theres not a whole lot that can plug an exhaust system,

a bad cat being # 1
a crushed pipe # 2
a banana # 3

Don't know why theres even a test for this. Tap your muffler with a hammer and listen for loose crap...yes?.. bad muffler. The cat?..just take your rig to an emissions testing station, if you fail, there you go.. or pull you cat off and look at it..bad cats also get REAL hot..like glow red gates of hell hot. A high flow cat is worth the money too.