1998 Ford Ranger 4WD High and 4WD Low lights flashing

Have a '98 Ranger 4WD, 4.0L V6, with 122,000 miles, manual transmission. The 4WD High and 4WD Low lights flash 6 times, then seem to go 2 minutes, and flash 6 times again, until I shut off the engine. Let it cool, drive it another day, and it may not happen. Seems to be intermittent. No noticable sounds or smells, or performance change. 4WD High still functions as it should, and shifts back to 2WD normally. 4WD Low has never been used.