1997 Ford Ranger right bank lean

I'd look for any possibility of an intake manifold gasket leaking in that bank of cylinders you're having the problem with.

A dirty MAF (you cleaned it already) would likely cause lean codes on both banks of cylinders.

Even though you already replaced the O2 sensor, I'd swap left and right O2 sensors just to see if the code follows the sensor, or if it still remains the same. If possible, get yourself a good scanner and actually read the values of the engine bank giving you trouble.

Um, another thought: I don't think Ford trouble codes say "Left" or "Right" Instead, they say Bank 1 and Bank 2. Believe me, I've made plenty of dumb mistakes before: Did you replace the correct bank sensor? Maybe you replaced the wrong O2?

Two ideas.

Pressurize the cooling system several times with a cold engine (up to about 8-10 PSI) and see how long the coolant lines hold pressure. If the coolant system integrity is good, pressure should hold. Then you may not have a coolant leak. Eventually leading to another problem, the Bank Lean code. You can be creative with hoses of various diameters and ways to pressurize the system; The tricky part is keeping the pressure in there to see if it's leaking out some other place.

Check your spark plug colors. If the plugs are old, get yourself a set (6) of cheap new plugs, run the engine for a few miles, then pull the left & right bank plugs out and compare the color very closely. The lean bank should have a lighter color than the "richer" bank. But you need to run the engine primarily at the RPM and load where the engine is running lean. If you have the ability to get a "Freeze Frame" snapshot of operating conditions when the code was set, this is the operating condition you need to concentrate on.
For example, if the Freeze Frame says 35 MPH, Engine Load 40%, and 1850 RPM. Try the majority of your driving (traffic permitting) at these parameters and try to get the code to set again. Then, as soon as possible, shut the engine off and read the spark plugs. Running the engine (for example) at long idle if the Freeze Frame says 35 MPH defeats the purpose of getting the spark plugs to coated with the color because then the plugs will be coated with carbon deposits from idle running, not at the parameters where the fault occurs.

Reading the spark plugs further verifies a "lean" condition and may rule out a faulty or lazy O2 sensor.