I've got an 03 Limited with the V6 and at 112,000 miles I'm going through something similar.
My front manifold broke at the flanges and $700 later it is fixed. That was me buying the part through Pepboys and having a local muffler shop do the labor.
My back manifold is also leaking, probably due to missing bolts and a small crack, but unlike last time, this time I ordered the part from Partsgeeks and saved about $85.
The replacement manifolds are made by Dorman, (my muffler mechanic recommended them) and the part numbers are 674-830 and 674-831. Cost is about $365 each (buying from any place else such as Pepboys, Autozone, Advance Auto Parts, or O'Reilly and the price is $439 minimum, more if it has to be special ordered and guess what brand they order.... yep Dorman.
The rear manifold is harder to replace than the front as other parts have to be removed to make it easier to get out.
If the main cat was clogged that happened because the O2 sensors were saying that the cylinders were burning lean and so more gas was dumped into the engine making the exhaust even hotter and melting the precats which in turn ruins the main cat.
One last thing..... neither of these manifolds are CA legal.