This sounds like a cat issue to me too.
A good indication is really poor mpg and loss of power.
The power loss symptom is sometimes hard to determine.
If you have a light truck with good hp it may not show up easily.
One way to determine real power loss is to load the truck up, and hit a long steep grade. If u are depressing the gas and downshifting way too much, this is strike one.
The easy no high tech tool test before pulling cat is to remove the o2 sensor before the cat, tie it up, and hit the hill again. If u run right up the hill, strike 2.
Final test, pull the exhaust and look at the inside! It usually shows broken or chunked up pieces. Strike 3.
Time to post on TW for a used cat, hit the junkyard, or buy a new high proformance cat.