principles of ABS questions

["Andrei Broder" <broder@pa.dec.com>]

I understand that ABS works by measuring angular velocity -- in a
nutshell the braking pressure is reduced at the wheel that rotates
slower.  My question is, what happens when all four wheels are locked?
That is, how does the ABS computer "know" whether I am sliding or fully
stopped on a icy, 20% slope, driveway?

A maybe related question: it seems that making ABS work with fulltime
4WD is more difficult; on some part time 4WD, engaging 4WD disconnects
the ABS.  In my 92 100, blocking the rear diff disconnects the ABS --
of course there can not be any speed difference for the rear wheels,
but couldn't ABS still work wrt the front?

TIA, - Andrei