Yes, actually, it's rather different.

1987-1991 (and probably earlier years too, I just can't remember the real old stuff) used a vacuum-operated front axle. When you pull the 4WD lever, it operates that vacuum switch on the transfer case, which then operates a vacuum motor on the right side axle shaft (called Vacuum Central Axle Disconnect, or just CAD here on this site), which THEN operates a little switch for the light on the dash....



The dash indicator switch is located on the passenger side fender behind the coolant pressure bottle. It's supposed to operate when vacuum is applied, so if it remains on with the vacuum lines unplugged from it I'd say it's bad.

 

THANK YOU for clarifying the system for me.....now I have a good foundation to diagnose!

 

It's sort of a neat setup. Up by the expansion bottle on the firewall is a blue vacuum switch with one line to it. The "disco" on the axle has three lines. One to pull the sleeve onto the splines to make the axle essentially "one", one to pull that back to let it freewheel. The third won't provide vacuum to the 4-wd light switch unless that sleeve has moved over, or stop the vacuum unless it has moved back. A sort of "bullet-proof" system, for letting you know weather it's engaged or not. That feature was eliminated soon after 90, (By 94?).

A big, common problem with those is the rubber deal with two lines on the disco gets oil soaked and funky and falls off easily. A large loose zip tie around the whole axle can help allot keeping a limb or something from knocking that off. (or one could redo the lines altogether)

The faulty fork casting was re-designed by 89. (earlier would break sometimes), mine did on an 84. I just had it welded and it's fine right now.

Edit added. Dummy me! I skipped over thinking Salad had just posted the diagram. Suppose redundancy is better than the alternative!

 

THANKS for the great explanation on how my system works! Pretty sure 4X4 is all working, but I'll have a peek underneath to see if there is any concerns there.