help!! low box not engaging

[moggy1968]

While stuck in the bomb hole on the strata florida I discovered that when the drive train is under a bit of duress, the 4wd low is disengaging. The lever doesn't move, it just makes a clicking noise and there's no drive to the wheels. I don't reckon it's the free wheeling hubs because with the front locker engaged (which I can feel through the steering) there's still no drive to either wheel. I'm thinking something with the transfer box.
It seems fine in 4wd high and does appear to be engaged in 4wd low on the flat, it's just when the front axle is having to work a bit
Any ideas?