I just had my 98 GMC 3500 looked at for possible transmission problems. They tell me my transmission is fine, but the cats are clogged. Is this possible with 84K miles? If so, what would the symptoms be? The truck is kind of a dog and a gas hog (454). It also seemed to miss a little under a heavy load while I was out plowing. They are pretty expensive for this truck so I want to make sure I get the diagnosis right.