I have determined that the horrible exhaust leak noise that my truck makes when its cold is the passenger side cat. It's not leaking but it rattles like crazy! So my questions are: is it ok to replace just one? And can I, should I, use an aftermarket universal one? Thanks!

 

Just have an exhaust shop cut out the bad one and replace it with an aftermarket. I did that two years ago and it has worked fine. People will say you need to find the source of the problem that caused the cat to go bad but, sometimes they do just go for no reason. That being said make sure you dont have a blown head gasket or that your running it with a bad missfire.

 

If you are at or close to 100,000 miles, I would replace both sides with some Hi Flow style cats.