Since you have them out, it won't hurt to clean them up a bit with some green scotch brite and spray them off with some break cleaner. Are you getting codes right now? If so, what are the codes? That will help us point you the direction where to go. One thing of note, when I have an O2 sensor code on my 2011, it turned out the driver's side catalytic converter was going bad. Research on the specific code on sites dedicated to Ford Rangers indicated it was a cat problem and not a senor problem. Also, just before the code was thrown, my gas mileage suddenly became much better than it normally would be. A big indicator something was up. I changed the cat, clear the code and the problem was fixed. I also chose not to go with an OEM cat but one from Rock Auto (Walker I think). It was much cheaper than what the OEM Ford one was and hasn't been a problem.