My Cat is falling apart and I have a new one to replace it. While I am at it, I figured I'd replace the O2 sensors (upstream and downstream), but I'm curious about something: is replacing O2 sensors a straight swap and done affair? Is there some computer work that needs to be done? What about the Cat? Just unbolt old and bolt on new?

 

Yes, to all

All O2 sensors on Rangers were the "same", only difference in part numbers has to do with the wire lead that comes attached to the O2 sensor, i.e. 6" long wires, 12", 18", ect........
Plugs at the end of the wires are the same.

O2s can be easy or hard to get out, since you are replacing them you can just cut the wires at the O2 end(unplug connector first) then use a deep 7/8" socket, or 22mm socket
All O2s use that size wrench(7/8" = 22mm so either will work), some O2s are located so you can't use a socket, if wires are cut you can use closed end wrench, or Flare nut wrench, even a crows foot Flare might be needed on some locations, lol

Yes, you can "just unbolt" old Cat and install the new one, bolts are often rusted in place so you may need to cut the bolt, so expect to replace them as needed.

No "reprogramming" is needed
If there were O2 sensor issues the computer will sort things out over the next few drive cycles and change the long term fuel trims(LTFT) number to reflect the new O2 sensor readings
If you disconnect one battery cable while you work on the vehicle, always a good idea, then after its hooked back up the computer will "look for changes" automatically

 

Awesome. Sounds like an "easy enough" unbolt/unscrew job. I like it. Thanks.