Thanks everyone.
I actually don't seem to be able to replicate the problem (which I didn't actually observe).
When it 'occurred', my boyfriend dropped me off and I went on a hike, while he went exploring an Indian shelter. When he got back, he only told that me that the 4x didn't work and the rear wheels locked up. I didn't test it at the time, because we had another day of work that didn't need 4x and then we had a 3 hour drive back home and I didn't want to screw anything up before we got back home. I just emailed him for a more complete description and this is what he said " I was driving in 4x4 for ca. 30 min on the easy dirt road, and didn't notice anything. I noticed the rear wheel locking up first when I accelerated to get the car up an incline to the (cement) platform to park. Only when turning the car around on the platform I felt the resistance from the rear wheels. I got it out of 4x4 (after some jiggling) which also released the rear wheels. Then while stopped on the platform back into 4x4, and the rear wheels locked again. Got out of 4x4 again, and that was it.
picture of the car on the platform and the marks the tire left http://www.smugmug.com/photos/i-8M2gq9w/0/L/i-8M2gq9w-L.jpg
My testing:
I marked the tires, and put it into 4 wheel drive (not a fan of the button), and drove forwards, checked the tire marks, and each tire had rotated equally. Drove backwards in 4x and checked the marks again, again each tire had rotated equally. I tested the on-the-fly shifting and it didn't cause any obvious problems (although I never like to do this, even when 'allowed').
This car is a trooper, it's been garaged/parked for most of 15 years with a few oil and belt changes, then two years ago I pulled it out and drove it up to the trailhead for White Mountain peak at 11,000 feet, I thought it would die on me but it didn't. Got it out of mothballs again now, and it's got a little bit too much exhaust, and I had to replace all the tire stems, but it seems to be working well.
I'm thinking that it just didn't like turning on the cement platform. (the location was a sandy dirt road into a railroad siding where you can park to hike and access the Indian shelter).