My truck is a 1991 v6 4x4 automatic transmission. When I switch to 4x4, the idicator light under the tach, starts blinking very rapidly. I tested the drivetrain and the 4x4 IS engaged, 4 wheels have power. Somebody told me it could be a vacuum leak, or some sensor on the 4wd system. Any ideas???? thanks

Check the connections to the light itself. They could be a problem in the wiring, possible a cross short.

Check your owners manual. Blinking should be indicating a problem, quite possibly a vac leak if you have the ADD front axle.