Don't have the codes, but I can think of a couple possibilities:
1. You said it's burning a lot of gas. Sounds like either you're misfueling it, or it needs an engine transplant.
2. Are you sure it's firing on all cylinders? If one is missing, that'll put a pile of "free" oxygen into the exhaust. Since the oxygen sensor is downstream from the manifold, it'll interpret this as a mixture that's too lean, and crank up the fuel quantity. Next, the cat gets a load of raw fuel, and the oxygen to go with it. Guess what that means? Lots of heat from when it reacts the two of them, which shortens catalyst life. 1996 is a bit too old to have a dual oxygen sensor setup (one before, one after the cat), so you've probably got just one (before the cat). Seriously overrich mixture and one cylinder not pulling its weight (you didn't mention I4 or V6) will both result in sluggish performance.
3. For a few years, GM offered FFV as a no-cost option on its small pickups (due to regulatory incentives, every FFV sold gave a big bonus on CAFE). If the fuel sensor (to determine ethanol content) is faulty (reading too high an ethanol content), it'll enrich the mixture. This one's a long shot - rich mixture alone shouldn't damage the oxygen sensor or the cat.
4. Check your vacuum lines and any vacuum accessories (power brake booster, vent doors, charcoal cannister purge, etc.). If you've got a leak, it'll bring in raw
unfiltered air, the oxygen sensor will see this as a lean mixture, and add fuel. Again, it's a long shot - the system would be correcting a genuine lean condition, it shouldn't hurt the cat or the oxygen sensor, and you'd be getting too much power at idle and part throttle (leak gives too much air, system compensates with too much fuel).
I'm not a mechanic, and I don't play one on TV, but let me know how this turns out.
