it sounds like your cat is finished.
if your truck is cold (like sitting over night cold), will it maintain highway speeds right away (while still "cold"), and hold them steady? and then once it has warmed up it gets really sluggish with no power?
drill a small hole right in front of your cat (3/8 is fine), and see how your truck drives then. you can also check what the temp of the cat is to see if it's plugged, but that isn't alway a sure fire way to tell.
as an example of that........the cat on my '98 was finished, but the temp was ok across it. when i pulled the cat, i found the front row of material had become clogged, then broke up into pieces, clogging against the second row, and was slowly choking off the exhaust.