1998 Ford Mustang "Diagnostic Question"?

your lean codes, most likely dirty airflow sensor. its locate in the air intake hose between the air filter and throttle body. you will need tamper proof torx to remove and clean it. 2 screws hold it in a plastic housing, its the black rectangular sensor, once removed, you will see two wires, very fragile and probable coated with black fuzzy stuff. carefully clean them with electronics cleaner or brake cleaner, let it dry completely before reinstalling. this could be at least partly the cause of the shudder too, but in case its still there, replace the spark plug wires (I did not see where you said you did that) and go the the parts store and get a trans additive that stops torque converter clutch shudder and that should cover all the bases. lifters are not normally noisy on V6 mustangs and most ford injectors are noisy.