when i did mine the one that was closer to the turbo was the cat converter... "was" is the key word there but to answer your question..... no the aero wont replace both. aero has a aero xl which is a resonator which i beleive is just for sound reduction and that is the model i have, i love the aero! i had another straight thru design muffler that made my truck sound like a rice burner so then i got the aero and it sounded alot better... well to make a long story short i have both of them on there now to bring noises down even more and i am 100% happy w/ it.

 

The one in the front is a resonator, and the one in the back is your muffler. I cut them both out, straight piped where the stock resonator was, and installed the resonated version of the Aero ( the XL version ) back where the stock muffler was. I couldn't be happier !!

You have a 98.5 and you have a 53 block? I didn't think the 53's came out till later.

Two other things that i didn't see on your list. Clean / replace your air filter, and clean your battery terminals and ground wire connections. Faulty grounds have led to erratic shifting.

Have you changed your diff fluids yet?

 

I confirmed the 53 block after I read about it here. I got a K-N insert filter. I know most guys here don't like em. I'll attend to cables. I already found a loose ground by the bumper bracket, drivers side. Thanks for the muffler info. That was exactly what I wanted to know. I'm gonna do the diffs. Does this year require the friction modifier in my limited slip?

 

yes, you need friction modifier for your rear Dana axel. you dont need it for the front.

 

I've got a K&N and it's OK with me. Perhaps while you are servicing it you might as well yank the silencer ring since it's right there anyway.