4x4 Light Flashes Above 25mph

[richarddews]

I have a '91 4x4 4.0 4dr. Have already replaced auto hubs with rebuilt ones from ebay, and they work great. In this recent cold weather, here is what my 4x4 system has started doing: Pressing 4x4 works great, engages without problems and stays with appropriate indications until 25mph. Almost exactly at 25mph, the 4x4 bottom dash light and little orange led next to the 4x4 switch start flashing erratically. They ultimately both go out at even higher speeds, while flashing briefly on rare occasions. No unlocking or disengaging sounds from drivetrain, through. Once I slow to that magical 25mph, all lights come full on again. What must I check/change? Thanks!