Intermittent front drive train sound

I have a 1991 4Runner with auto locking Hubs and for several months an intermittent noise coming from the front drive train. The noise sounds like the hubs are trying to engage (metal gears slightly rubbing together) out of sync with the rest of the syncro system. Most folks think the noise is hub related. During the periods when the noise is happening I seem to hear a relay under the dashboard clicking rapidly.

Toyota assessed the problem and adjusted my linkage, but the sound returned within 5 days.

I have determined the following;
AT 20 -40 mph the noise goes away if I turn the key off, and returns when I turn the key on. The engine does not have to be running during this time. The noise frequency increases or decreases as the 4Runner speeds up or slows down, not by RPMs if I go into neutral and rev up.

Another way I found to stop the noise is to shift into 4WD (High range). Once I shift back to 2WD the sound may not come back for a period of time. Sometimes several days.

The noise will also stop once I slow down to approx. 2-5 mph, at which time it feel like something has disengaged with a bit of a tug. Once this happens the sound may not come back for a period of time. Sometimes several days.

Any ideas on this problem or known solutions are welcomed. Thank you!

Mike B.