Cris S Gravel

Hello All
New to this forum but I have had my 2000 F150 4x4 for almost a decade now. My truck has the shift on the fly, electric 4 wheel drive, not vacuum operated but has electric shift motor. I've had an intermittent issue with the 4 wheel drive not engaging and what I mean by that is the motor does nothing when I turn the switch, I can hear some clicking behind the dash but 4x4 light does not engage and motor does not move. The weird thing is it seems to resolve itself for long periods of time but recently completely stopped working. I thought it was the shift motor so I just replaced that and it worked for awhile but the intermittent issue returned. Not sure if the switch on the dash may be faulty or relays maybe? Shorting wire could be a possibility. I was hoping others may have some insight on what my next move should be.