I don't own a Syncho (yet) but would like to offer a
different way of answering the question.

Todd Hill's reply indicated what I've always understood
about locking differentials and refers to a single axle,
either front or rear.  Given that, a vehicle normally
described as 2WD is really 1WD, because if one wheel is
lifted or on ice ...  (It would need a locking diff or
limited-slip diff to be truly always 2WD.)  A Synchro
without diffs locked is then really not 4WD but really 2WD,
because if both one front and one rear wheel are lifted or
on ice, there is no forward motion because 2 wheels will
spin.  What I always assumed a Synchro's diff lock did was
make the vehicle 3WD, because there is only one diff lock
(rear).  Hey, your chances are increased by yet another 50%,
which is not bad.  (Is 50% the correct number?  If a Synchro
with diff lock in has both rear and one front on ice, it
doesn't go, but if it has both front and one rear on ice it
does go.)

Also theoretically interesting (possibly) is the notion
there is no deception by the auto industry because in the
long run 2 wheels do provide drive on what is commonly
called a 2WD vehicle (but only simultaneously if both have
traction), but one wonders if a Syncho can be said to ever
effectively drive all 4 wheels simultaneously, since as I
understand it the viscous coupling never couples until there
is evident loss of rear traction.  For a Synchro then, 4WD
must just mean that over time any of the 4 wheels could be
driven.