I don't know how your truck is set up, but I know that the rear O2 sensor on mine sits about 4 inches downstream of the cat on the exhaust tube. You spray some penetrating lube, wait a few minutes, take out 2 bolts, replace the O2 sensor and supplied crush gasket, then put the whole thing back in. The front one requires a little talent and some limber wrists (since it's far up on the headers), but it's definitely not too hard.
The only shield I see on mine is the heat shield on the cat, which you shouldn't even need to get close to when changing the O2 sensors. If there's another shield down there, it shouldn't be too hard to unbolt....
Maybe your '03 has a different setup, but I was able to replace both O2s on my '99 in about 30 minutes without a hitch. Take another look, and write back if you run into trouble! <img src="/forums/images/graemlins/cheers.gif" alt="" />