Clogged/bad cats?

[MSFoster]

I recently sent me 00 expy to the shop and had a new transfer case put in. The check engine light was on so I had them check it out while it was there. They said it was running lean and that my fuel filter was clogged upso I told them to change it. the light has stayed off but now two weeks later I am still getting bad gass mileage 10mpg and it now shudders when accelerating. Also seems as though it has a bad miss fire when idling. I was thinking it might be clogged cats but not sure. Any insight would be much appreciated. also it has 174xxx miles on it and is 5.4L 4WD jic you needed to know. Also if I do have to replace the cats I want to only replace two of them and it has two O2 sensors on both sides 4 all together will I have to do a tune to it or will it be fine just putting two high flow cats in and deleting the other two? I have very little experience on exhaust.

 

[walnuts4x4]

the misfiring and shuddering I will place a bet saying you have a COP going bad... when they have a low grade misfire they cause those symptoms... shudder,miss, bad mpg,... best and cheapest route is going with COPs from ebay from either global automotive or UNEEK.. all 8 shipped for under 100... otherwise cops are 25-44$ a piece... for plugs I would only use motorcraft of autolite double platinums.... otherwise these motors don't like other plugs...

the reason no CEL is on is because of the low grade miss... needs to get worse to trip the CEL... I fought this problem last summer...

as for CATs.... I did a rear cat delete only pipe... and nothing was changed... but if you wanna replace all 4 I would get 2 high flow cats since these motors need some back pressure....

also more info can be found in the link under my pic... soon both sites will be merged here and only one site