Hi Richard,
OK, this sounds as though you have a Diff Lock on the front axle and
then possibly free wheeling hubs - which is the standard setup on 78s.
The switch on the dash will engage/disengage the diff lock and then the
hubs are manually locked if the wheels start to spin - although I'm not
entirely sure.
Is this a European or Japanese model - if so please couldd you email
me your VIN so that I can look it up on the parts system.
If the front diff lock isn't engaging it could well be the same as
you get with 80s - basically water gets into the diff lock actuator
stopping it from working - you will need to remove the actuator, strip
it down and then clean it out and relubricate - that should get it
going.
As for the free wheeling hubs - they also will need removing, stripping
down and then re-assembling. I don't recall any brushes in them
although there are breakable components in there that could be causing
a problem.
--
Regards,
Julian Voelcker
Skype: julianvoelcker
Cirencester, United Kingdom
1994 HDJ80, 2.5" OME Lift