Popping out of 4 wheel drive?

Was wheeling in the sand a couple days back and for the first time I noticed that when the engine is under heavy load the front axles seem to disconnect intermitently... well let me clarify that, the 4 wheel drive indicator light goes out. When the load on the engine is reduced it comes back on. No shaking or banging like I'd expect if it truely was disconnecting but I don't really know. Like I said, its when I am mashing down on the throttle to get over a dune. Any thoughts?

Vacuum

You have a vacuum leak somewhere. You probably have problems with your defroster also. Check the canister behind the front bumper passenger side as well as all the hoses and connections. Good luck.

Defroster? You mean the one for the rear window or the one for the cabin? The one on the rear glass is inop but the other is fine.

By the canister do you mean the charcoal one for the gas fumes?

Thanks

The defroster issue: When vacuum fails, the heating system will default to defrost. If air only comes out of the defrost vents, not the dash and floor vents, it's a vacuum symptom.

The canister: This is a plastic "blimp" located behind the right front bumper (concealed by a plastic flap). The lines to this run beneath the battery tray, and corrosion often eats them up. If there has been any impact to the bumper, the canister can crack, and even if not, as it ages, it may become brittle. Check for broken mounting feet, which often chunk out a tiny hole.

While at it, check the rubber connector on the vacuum shift motor itself (it's on the right side of the front axle). It can get soft, and start to leak.

If you can't find the problem in the lines or the reservoir, you may have to check the vac. motor itself for a leaking diaphragm. The vacuum switch in the transfer case can also fail. The Vac. motor is very expensive, and the switch is fairly cheap.