There is no linkage for the front axle engagement....it will kick in when a certain amount of slippage occurs between the front axle and the rear axles. If memory serves it was 4 or 10%. When the rear axles are trying to over run the front axles, the sprag automatically engages.
To test if the front sprag clutch is working, you only have to jack up one front wheel. You should then be able to turn the driveshaft one way (as if to back up), but if you go the other way the shaft will lock up. If the shaft will turn freely in both directions then the sprag clutch is buggered.
Both rear axles should be engaged all the time. If you lift one wheel and try to turn it, it should not turn. You would have to lift a wheel on each rear axle to be able to rotate the wheels. If you can rotate with only one wheel raised, then you have a problem. It could be a broken axle shaft, or a broken differential. These were quite common on these trucks when I used to work on them. A trip as delicate as a road test could result in a broken diff.
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