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You should always include year of your Ranger in posts

Most likely there is a loose connector on the cluster wiring

I assume a 1994 or earlier Ranger since newer models just had one light
These 4WD modules used one wire to turn on both the button light and the cluster light, since button light works all the time then modules wires are tight, which leaves clusters wire as the problem

The instrument cluster 4WD lights were there because Manual shift 4WD transfer cases were more common in 1994 and earlier Rangers, and these, of course, had no buttons.
There was a switch on the manual transfer cases that could turn on the 4WD lights for 4hi and 4low

These lights do not effect 4WD of course, they are just indicators, but with the electric shift, like you have, the 4WD light will start to flash if you try to shift into 4WD and the shift motor doesn't move, this can indicated blown shift motor fuse or failing shift motor