Honda 97 Passport 4wd does not engage

I have have a 97 Honda Passport but the 4wd does not work. When I start it,
all dash lights including the 4 wheel drive light come on, so I know the
light is ok. But when I engage the 4 wd the light does not come on and the
4wd does not work. The shifter seems to work ok, but no 4wd. When we put
it up on a lift, and engage the 4wd lever, the shaft to the front
differential turns, but the axles do not turn. The locking mechanism for
the axles seems to be attached to either side of the differential. Was told
that there is a vacuum solenoid that locks the axles that may be the
problem, but don't know where it is located. Any help would really be
appreciated!!