Could be a number of things.....
First, as you have mentioned it, the viscous coupling, jack up one front wheel so it is clear of the ground, put a wheel brace on one of the wheel nuts and with the handbrake securely one, it should turn slowly round. If it is locked solid then the vehicle's viscous coupling is knackered causing binding whilst driving in sharp turns. Replacement can be done in situ, and not too bad a job.
Secondly, wheel bearing, possibly, but I would say unlikely in this case, but is certainly the easiest to check, jack the front wheels up and try rocking them side to side an up and down, any side to side clunks is almost cetainly wheel bearing, a collapsed and therefore locking up bearing should be pretty obvious(!)
Another thing... could be a CV joint broken/breaking up/very worn, these click from the front axle when on full lock as they are under the greatest stress then. Checking them is a complete hub disassembly, and not a nice job, really mucky.
You could, I suppose, have low fluid in the reservoir, but I guess you have checked that
The viscous boxes are great btw, on road they drive much nicer, I went to an LT230 box for better gearing, and it does drive quite differently.... and chirps tyres at a standing start