No auto-engage 4WD operation w/engine running

I've got a 2001 Ranger 4.0 with automatic locking 4WD hubs. Recently the hubs will not engage when the engine is running but work normally when the engine is off and the key is in the "acc" position. I've checked the fuses and wiring all which appear good. Has any anyone seen this condition before and, if so, what was the fix?

How do you mean the hubs work in OFF or ACC?

Sorry I don't have an answer for you but, i also have a similar problem starting. i have a 2001 Explorer sport 4.0 4X4 with auto hubs. Recently the 4wd clicks on and off engaging and the disengaging while driving down the road. Also with the vehicle running in park it does it too. Seems to me my issue is electrical as i can hear the relay clicking on and off behind the passenger side dash board. I was wondering if you have a problem with your abs sensors. no abs light but a constant chattering pedal when stopping as if you were sliding.

I put this problem on the back burner for a bit but had some time to dig back into it. I checked all the circuits for shorts and opens, and was able to eliminate the transfer case motor. This left the 4WD control module which is a printed circuit card in the passenger side kick panel. There wasn't much in the way of mulitmeter testing info so I decided to try one from the junk yard for $40. This fixed the problem. The old card has no burn marks or odor so I'm not sure what failed on it but I have heard that it is sensitive to voltage spikes like changing out the battery.