Front End Of 4x4 Locked Up

Something locked. Here's the sequence of events. I drove up the steep, unpaved entrance to my house, about a quarter mile, in four wheel drive in my 4x4 Mazda pickup. At the top of the drive I shifted out of FWD and pressed the dash switch which electrically disengages the front hubs. Then I pulled in to my dry cement parking area and reversed making a ninety degree turn and parked. The next day I started up the truck, shifted into first, started to let out the clutch and it wouldn´t move. At first I thought the parking brake had frozen for some reason and gave it some gas to break free. Then I realized the rear wheels where spinning in place and that it was the front end that wouldn´t turn. At this point I should have jacked the front end up so at least one wheel was free in the air but I didn´t. Instead I shifted the transfer into 4 Low and started to ease out the clutch. There was a loud clunk and it broke free. I shifted back into 2 High and went on my way. I later shifted into 4 High on some steep loose sections and the 4x4 system seemed to work as normal nor were there any strange mechanical sounds. So what happened? How could the front end have locked up solid or was it from the transfer case forward? All I can figure is the front hubs and 4x4 didn´t disengage during the short distance I was in reverse and the short ninety degree turn somehow bound up the transfer case. Any ideas?