1999 Ford Expedition, all four tires locked up yesterday, ten minutes later tires freed up and moved fine?

I heard a loud noise and then all tires felt like they locked up and the car skidded to a stop. Two people had me put the car in neutral to try to push me off the road, tires would not move. About 10 minutes later, we restarted car and put it Neutral and it still wouldn't move. Put it in Drive and it moved fine. No explanation for the lock up and dealer can't seem to find an explanation for it. I'm terrified to drive it without knowing what might have caused it. Any ideas?