2001 SIlverado, 5.3L, 3.73'*, 4WD, 92,000mi.

When I first got this truck a few months ago, I had to watch my right foot, because the rear tires would easily break traction on just about anything. Climbing small hills was never a problem, I give it more gas, it kept on chugging.
Something is different now...

It doesn't take off like it used to. I haven't added anything, but it feels like I all of a sudden hooked onto a trailer with this truck. It doesn't pull away from a stop like it used to, and small hills choke it. It doesn't throw any codes. I got the fuel system including injectors cleaned. It does ok when I force a downshift, and it takes off good once the rpms go up. But it just seems like I've lost the low end power somewhere. I drive the same roads to work everyday, up and down a lot of hills, so its a noticeable difference.

Only thing I can think of that I've seen before, is where the catalytic converter on my sisters 01 Regal was coming apart inside, and clogging up airflow. Ran ok if you kept the rpms up, but wouldn't climb hills, wouldn't respond to throttle input. You could put your foot down, but it just wouldn't go faster unless it forced a downshift. It never threw a code, and it baffled me and the mechanics until they finally pulled the converter off.

I don't know how common this is, but when I first got the truck it was fine, but then I started taking it off road (nothing too serious), and now its having problems. The truck is in great shape, and only has 92k, but what are the chances that the converter is breaking apart?

Brakes are good, they aren't dragging, alignment is good, fuel is good. Air filter is good.

If you have a rubber mallet or piece of 2x4, smack the cat converter and see if you hear anything rattling around.

Another cat test would be to remove the upstream O2 sensor and install a pressure gauge in the bung hole. Check the pressure, reinstall O2, remove the downstream O2, repeat test. This will tell you if your cat is clogged. To clean cat, you need 2 more people. Ensure all four feet are held onto, take funnel and ram into mouth, pour Ester oil into funnel, remove funnel, run like hell. Oh wait, sorry, wrong cat.

cat is suspect, some other things, did they check the fuel pressure when they cleaned it? how old is the filter? you have a scanner that reads live data so you can check fts and maf values?