We were staying on the beach at Holly Beach LA  and exploring with the trucklet. It was not particularly difficult terrain but the sand was soft in places. After a couple of hours we got back on the road, unlocked the hubs and went to 2wd.  We made a left turn, went 200 yards then a right and something jammed, I believe it was the right front. I checked the u joints and front end and saw no problem. I tried backing up with no improvement shifted the trans up and down. Tried D and the transfer case in neutral and it spun free which ruled out the regular trans. About then a couple of locals showed up and offered help. About then I tried it again after shifting the transfer case back and forth and it worked fine. A local guy who wandered over said it was stuck between gears and his pickup had done that too. Since then it has been towed a few hundred miles, driven in 2wd and run for about 4 hours on Padre Island in loose and hard sand without a problem. Should I worry? Can they get stuck?  Headed for Great Bend National Park to explore the backcountry and I'd hate to get stuck.

Having read your post twice, I can't even  figure out what the "symptom" you're asking us to diagnose is.  Did the vehicle stop moving, become immobilized in some way?

If you were in 2WD with the hubs unlocked, as you say you were, then the only thing in the front that could jam and bring the vehicle to a halt would be one of the wheel assemblies, if you were actually in 4WD with the hubs locked, that would be a completely different story.

Driving on a hard surface in 4WD can cause the transfer case to bind, and it can happen in a very short distance, if you're turning corners - typically it will make the vehicle "struggle" to move, and you'll feel the wheels/tires fighting - I guess if it's bad enough it can bring the vehicle to a complete stop, it can certainly break the drive train, but based on personal experience, you should feel it long before it gets to that stage.

Sorry if I was not clear. I was in 2wd and the wheels partially locked

Partially locked as in not straight ahead and not full lock (left or right) - or - partially locked as in unable to rotate freely?

In either case, once it's 2WD and the free wheel hubs were unlocked, the 4WD mechanisms are completely disconnected.

Partially locked as in not straight ahead and not full lock (left or right) - or - partially locked as in unable to rotate freely?

In either case, once it's 2WD and the free wheel hubs were unlocked, the 4WD mechanisms are completely disconnected.

Unable to rotate freely.  Drivers side front wheel left black marks on pavement but did rotate somewhat.
 CV Boots are intact. I had the front dif out a month ago and the CV's seemed OK then. No noises when rotated. Front bearings spun easily with no extra play.  At that time the front universal joint was fine also.
  It felt like  something stuck in the front diff or wheel assembly but I couldn't find anything. Admittedly I could have missed something since I couldn't get far underneath. Nothing fell out once I got it going though.
  I am now at Big  Bend National Park and have racked up hundreds of miles  off and on road without a problem.  I am just assuming that I picked something up on the beach and it jammed somewhere and crushed or  fell out in pieces.   
 If you ever get to Texas try Big Bend. There are hundreds of miles of rock and dirt  roads and tracks without restrictions.