2000 Mercury Mountaineer Intermittent clunk

I have a 2000 Mercury Mountaineer, 4.0, 4 wheel drive (not AWD) with an intermittent clunk. The severity varies from slight to moderate and feels similar to putting the transmission into drive from neutral at a high idle. The conditions under which this occurs are as follows: 1) temperatures below 30 degrees 2)slushy or snow covered roads (has not occurred above 30 degrees in rain or wet conditions) 3) steady rate of speed usually between 1700 & 2300rpm (not under acceleration) Several technicians have test driven the vehicle and only one has experienced the problem. This has been an ongoing problem for a few years. Early on a diagnostic code for front wheel speed sensors was indicated. Both were replaced. This was the only time any code was present through computer diagnostics. The transmission was rebuilt at 45000mi. Other attempts to correct the problem included flushing both front and rear differentials and transfer case.
My selector has three settings which are auto, 4x4 high and low. I was advised to try to select the 4x4 high option and when I did so, the problem did not show up, however when I shifted it back to auto it did not recur until a separate instance of the same conditions were present/ I have not tried the low setting, due to  the high rpms needed to sustain highway speed. All four tires have recently been replaced and are the same brand, size and tread depth.  
I have owned the vehicle since it was new, and did not experience any problems of this sort until it was two to three years old. I know I have driven in conditions where there was not enough accumulation of slush or snow to cause resistance or traction problems. I wonder if there are any  sensors on the drive train that could be malfunctioning from the combination of both cold and minimal snow or slush, perhaps even 1/4 inch or less, as I feel this is far more than the normal transferring. I experienced one instance where it was severe enough to cause a fishtail.