You should post code definition with the code since you probably already looked it up
P1309 Misfire Detection Monitor not enabled
So not related to axle ratio change, its an engine code
This code is usually because CAM sensor is out of time with Crank sensor
2000 Ranger will have a 2 wire Cam sensor, 1998 and older used 3 wire and they are not interchangeable.
On the 4.0l OHV it is at the back of the engine, behind lower intake manifold on the block.
Computer also has these codes it can use:
P0340 Camshaft Position Sensor Circuit Malfunction
P0341 Camshaft Position Sensor Circuit Range/Performance
P0342 Camshaft Position Sensor Circuit Low Input
P0343 Camshaft Position Sensor Circuit High Input
P0344 Camshaft Position Sensor Circuit Intermittent
And it didn't use any of these, so CAM sensor is working, do not replace it
The computer gets basic spark and injector timing from the Crank sensor
The Cam sensor rotates at 1/2 the speed of Crank sensor so can be used to fine tune spark and injector timing, and if a cylinder misfires the cam shaft slows a bit and that is seen at the Cam sensor better than Crank sensor.
So Cam sensor is used to detect misfires.
Computer has to IGNORE cam sensor if its not in time with Crank sensor, Crank sensor is the main sensor for timing.
Engine can run just fine with no Cam sensor, but computer is letting you know P1309 Misfire Detection Monitor not enabled
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