All of the suggested VC tests passed indicating that nothing is wrong with my
Viscous Coupling. So the question remains: when I was climbing a snow covered hill
and the rear wheels were spinning (all be it slowly) then why were the front wheels
not also turning?

Lawrence Johnson wrote:
<<I raised both the front and back on one side and put it out of gear. When I
turned the front wheel by hand  the rear wheel turned smoothly. <snip>>

Malcolm Holser replied:
<<Jack up only *one* front wheel and try to turn it -- it should turn *very* slowly
with stiff resistance, but should turn.  If it turns freely, your VC is bad, if it
turns not at all, your VC is also bad. <snip>>