Why would 4x4 work in high range but not in low range?

Answer 1

Possibly bad transfer case.

If a GM compact (T) truck, consider a bad 4x4 switch.

If an Explorer you might have a bad contact plate, replace the encoder motor.

Because your transfer case might be shot it could be the bearings..auto locking? get manuals, it costs 100 and u need to get out, but it won't fail you. if electronic 4x4 switch, you need a new switch if manuel linkages, adjust linkages