Specs
2000 Cherokee Sport 4x4 (duh, right?)
4.0
AT
159k miles
Completely stock sans 30x9.5's =)

The rig is new to me, I know the family that owned it from new and they never abused it. And I don't think they EVER put it in 4WD either So I decide to test her out and take it up to Crown King AZ. It's a moderate Jeep trail with slight difficulty.
I'm running in 2WD for awhile, get to a point where I get to drop her into 4WD and off i go!
10 minutes later i hear a "klunk" one time out of the front of the rig and I'm like RuhRoh, that's not good. So i drop her back into 2WD, drive for another 10 minutes. No klunk. Throw her back into 4WD, hit the trail and 10 minutes later, "klunk"!
Mind you I was driving a simple trail in a fairly straight line (climbing) when I heard the noise...
Any ideas?

 

I'd take a look at your u joints. By what you've told us, I'd only assume they've never been replaced before

 

check all the joints in the front shaft and the front dif. you'll find out exactly whats causing it.

Also check the bolts, the yoke etc while you are under there it never hurts.

 

Mine does the same thing. I'd be interested to hear what's wrong with yours

 

I would definitely check your front u joints. Mine had the same problem and we "visually" diagnosed the problem. It turned out that the u joints were falling apart and needing new ones badly. Once we replaced them, no more problems!