Wierd thump from driveline

[kenora]

I was driving back from the lake on Sunday at about 90 kph in my 99 Explorer XLT 4.0 SOHC . It had been snowing, the temp was about 0 c (32 f) and the roads were pretty slippery, I left the 4X4 selector in AUTO. I was towing a small utility trailer but it was empty, trailer weighs about 400 lbs.
I noticed a LOUD DISTINCT thump from the undercarriage about 20 times on the 2 hour drive, at first I thought I was driving over a large expansion joint on the road or a branch or something because it was a single THUMP each time but unlike something you run over with first the front and then back wheels there would have been a THUMP (front) and THUMP (back)...this was a single event each time and had me worried. I stopped and selected 4X4 HIGH thinking that it might be the transfer case locking up momentarily then unlocking...is that possible, would it cause this noise and THUMP. If so should I have selected 4X4 HIGH as soon as I got on the slippery highway?
fwiw it didn't do it after I selected 4X4 HIGH but the surface conditions had changed to black ice by then as opposed to snow/ice covered with patches of black top (thats when the noise occured)..thanks for your help in advance.

 

[BrooklynBay]

Did the sound go away when you put it into 4Hi?

 

[kenora]

yes it did.. unfortuneatly the road conditions had also changed by then though...

 

[BrooklynBay]

Try putting it into auto again to see if you still have the same problem.

 

[kenora]

Its always in auto, the only choices are auto 4x4, 4x4 high and 4x4 low and no it hasn't done it again.

I was sure that someone out there had experienced the same thing and would be able to reassure me the transmission wasn't going to fall out on the road.

 

[daffy]

In my experience, I would say that it was your transfer case locking up becuase of wheel slip, makes the same type of noise, if you're going highway speeds, you probably wouldn't hear the small slip of the tire before the 4wd kicked in.

Mine does this on questionable roads as well. I'm gearing up for it as we speak, supposed to get an inch of snow on top of ice tonight...yea!

 

[K.S.MADMAN]

My 2001 sport doe's the same thing but i don't have the auto . but i can have it in 2wd 4-hi- 4-low it does it on snow cover roads not show much on ice . i had it to the dealer look at with no luck been that way ever winter does it more when it's really cold out i drive about 40 miles to work and it will do 4or5 times .ever time it snow's