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@Dammit
On the viscous locker- my understanding is that it contains a series of plates, attached to either axle. They spin in a fluid medium, and if the axles are spinning at the same rate then the fluid is at approximately atmospheric temperature.
If one set of plates spins at a different rate to the other (for e.g. if the rear wheels were spinning) then the fluid heats up very rapidly and this clamps the plates together, which locks the axles rotational rate to one another. The fluid then cools, and the plates unclamp when temp drops below a certain point.
If the vehicle is towed with two wheels off the tarmac and two wheels on then the fluid is going to get very hot, due to repeated heat cycles, and then (I suspect) overheat. Typically viscous lockers fail open, as it were, when they get too hot.
I’d be concerned that the centre diff has been subject to significant over heating, and overall has likely been degraded due to this.
I imagine a Scooby specialist could give you an engineers report to that effect (if I’ve recalled correctly how a viscous locker works!)