Transfer case locked

I have a 74 CJ5 w/ 252. the drive train is stock. 3 speed trans. Not sure what model trans or dif. anyway, I put it in four wheel and it went fine for about 25 yards then all four wheels locked and stalled the motor. After some cursing and muscle I was able to disengage the four wheel drive and put the transfer case back into neutral. It drives just fine in 2 wheel.



any ideas? I've never worked on a transfer case and this thing has never been services in the last 20+ years and driven only 500 miles in that time.



Thanks.