Front axle won't engage

[1969 GMC]

I took the GMC over to a friends house to help him move some boxes of stuff today, and got it stuck for the first time ever...it seems like his lower yard is nuthin but mud and gravel. Anywho, I locked the hubs, and popped it into 4-lo...and nothing happened. Since there were spectators (of course), I checked to see if either of the front wheels were slipping. Neither were. And they sure weren't gripping. Didn't check the driveshaft, though.
Does anyone know what could possibly be wrong? I know for a fact that the 4x4 worked before and after we rebuilt the front driveshaft, and both my hubs still act funny but work (gotta turn 9 times when you're supposed to spin 1/4 turn, I think). Could the hubs be shot? Any answers appreciated