Check the cats at night. See if one is beginning to "glow." If it's that bad, change it. It's clogged and extremely hot.

 

it might be either your primary or secondar mufflers, because the cats combine again into a single pipe, from the intermediate pipes I assume, then it splits after the cats into the true dual exhaust, through the primary mufflers over the rear axle, then through the secondary resevoirs under the trunk. It's a long road with lots of crawling on the ground to find the problem. Start by taking off the secondaries, and just put your hand up there to feel for pressure. Then just work your way back. I have the same problem with my '95 XJR, there's almost no pressure out of the left side pipe.