4x4 GMC Jimmy

Was wondering if someone could help. Have a 1994 GMC Jimmy 4.3l, when I go
to engage the (4x4 Hi) system, it does not work, and no light comes on,on
the dash switch. It will go into 4x4 Lo with no problem. I changed out the
vacuum module under battery tray and the cable going to the front diffs.
Still no success. I heard of the encoder motor going on these thing. Is this
something that can be taken off easily and where is it? Also there was
something of TCCM, what is this and where it is to diagnose. Any help or
past experience on this annoying problem would be very helpful. Thanks in
advance..Paul