Bob, I understand not replacing things without knowing they are bad. That is what I preach on here all the time. If your MAP is indeed reading what is really there, you either have a bad vacuum leak or a plugged cat. A simple check is to go to Autozone before you start and get a exhaust pipe union, then simply use a hack saw to cut the pipe before the cat. If it is the cat, you will see a significant difference in the vacuum. By the way, I like to use a vacuum gauge, instead of relying on the MAP sensor readings. If the cat isn't bad, just slide the union over the ends of the pipes and install a couple of clamps. to put it back to the way it was before.
The reason I think cat is that the insides of the cat get broken up into chunks. As you drive the chunks can rearange themselves, effectively plugging the exhaust, lowering the vacuum. You hit a bump, they move around, and presto you have flow. This is very common on vehicles with around 150K miles. Sometimes you can get under the vehicle and bang the cat with your hand, (when cold of course) and you will hear the chunks bounce around. The good thing is that if it is the cat, you will not believe the difference in power and mileage when you replace it.
Some XJ's have a mounting flange for the cat, in that case, just loosen the bolts to allow some leakage.
