I had similar codes a while back and being the lazy bum I am I just replaced all of them. I seem to recall it was only ~$150 or so for the OEM parts on amazon, and I think only an hour or so to replace them all. I didn't even need to put the truck up on ramps or jacks (though YMMV). I saw a mild (1-2) MPG improvement post install and the engine does seem to be more responsive, but that's probably all in my head.
When I was researching it what I found was that the O2 sensors just wear out over time, their expected life is only 8-10 years and these trucks are pushing 15-20 in most cases. They don't usually fail flat out, they just start slowing down their response and giving less accurate readings. This is common with all electrochemical devices, but it's very common on high temperature oxygen conductors like O2 sensors. My day job is actually working on these sorts of devices for fuel cell and electrolysis systems and we spec %/khr degradation rates for all of our systems and it's actually pretty difficult to get these things to run for >5 years.
The other thing to watch for if you replace the sensors and you still get the "too lean" code is cracked manifolds, they commonly fail on the 3.5 engines. If they start drawing in air it tricks the O2 sensor into thinking the engine is running lean and it'll feed more fuel to compensate. Replacing them is a bit of a hassle, repairs have only kind of worked...
