There is a vacum maudual that screw's into the transmission were those three vacume lines connect,a little ball inside there moves up and down to controle the vacume ingageing and disengageing the 4 wheel, i had the same problem,replaced it and wooks great, you will have to go through a chevy dealer to get the part tho, auto-stores don't carry it. good luck
Also as said above , plus there is a vac diff controller right under the battery, that vac hose could be bad ,and will no engage the the front diff i had to many problems with the ifs frontend i gutted it all and
put in solid axels hope this helps godbless
or RTM here
Fig. 48: 1989 4.3L TBI engine - federal altitude
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