How many miles on the truck?
Some exhaust shops will test your cats for free, but anyway, sounds like you may have ruled them out-it was a definite possibility. It's hard to help without being there, but I definitely have been here before with a few different vehicles. You have a good bunch of info, so let's run it down-I have disconnected the exhaust and it had no effect. It revs up when you first start it one time to 4000 rpms then after only 3000 rpms.