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Is the 4X4 light flashing on the dash? The front diffs use a vacuum solenoid to lock when when 4x4 is engaged.
The switch is on the rad cradle by the rad near the bumper.They do occassionally go bad.
90% of the problems I've come across is a hole in the tiny vacuum line on the front diff. If you go underneath, you'll see a small rubber hose the size of a washer line coming off the front diff housing, to a hardline which then runs to the switch. Inspect it for any cracking/holes/tears. I find they get really bad if the vehicle has had oil leaks for a while as it will deteriorate the rubber. All it will take is one small rock from the road or a chunk of snow to punch a hole in it.
If you have a vacuum gauge and a hand vacuum pump you can check all the components. You can lock the front diff with a vacuum pump to verify operation. If you check the line from the switch you should have steady vacuum. The only reason I suggest this, the last one I did had an intermittant problem. I could not visually see any problems in the rubber hose. I disconnected the line at the diff to put a vacuum on it, and sure enough, the vacuum dropped right off if I flexed the line. There was a tiny pinhole that was causing it to loose vacuum.
Hope that helps.
Now my memory is getting foggy, but I believe there is a switch on the transfercase itself (I want to say passenger side) that will send a signal to the computer to switch the pump on. If I remember correctly, there is also a 4wd fuse in the pass. fusebox (or am I thinking the XL7)
Are you getting any signal to the vacuum pump? When working you can feel it vibrate by putting your hand on it.
Firstly, I don't know what I was thinking, but yes it is an AIR pump, not a VACUUM pump at the front bumper. Since you haven't responded in a week, have you at least checked for power at the air pump?
The switch is indeed on the top of the transfercase by the 4x4 shifter. Once you access it, simply check for continuity in 4hi/4lo. I'm sure you could jump the pins on the harness to simulate a "4x4" on condition which would verify if the other components are working or not. The above will show you how to get to the switch.