I started at the top:
Inspected the relay (located above your right leg as you sit in the driver's seat). I pulled the black cover off the relay and then activated it just to make sure it was working, even though I could hear it click (it was).
Blocked the wheels, went under the 4Runner and inspected the shift linkage (had my daughter shift both levers with the car turned off). It all checked out and was tight (no missing pins/clips).
Located the actuator, which is mounted directly to the front differential, inspected two small vacuum hoses that connect the actuator to the metal vacuum lines (they were fine). I had to remove the protective plate on the underside of the vehicle to readily access the vacuum lines (four bolts).
Started the engine and tested the lines at the actuator for vacuum - the pull was almost non-existent. This was the first real clue to the issue. Be sure to have the emergency brake on and someone who wants you to live in the car doing the shifting. Come up under the car from the rear to avoid the spinning fan blade.
Noticed that the metal vacuum lines were connected into the system via rubber vacuum lines that run up the side of the engine. At the top, they connect into one blue and one red electrical vacuum control.
Upon inspection, found the protective casing had been partially melted away. Pulled the old vacuum lines out and found one of them melted shut and the other one melted open. Replaced both lines and everything came back online and the 4-wheel drive works perfectly again. By the way - when you replace the vacuum lines, be sure to note which goes to the red switch and which goes to the blue. Down below, I found a red and blue mark on the metal connecting tube, so it was easy to keep it straight.
So. My advice is start in reverse order from my repair! Weakest link is the rubber vacuum hoses running down to the actuator.
Sunday, December 28th, 2008 AT 6:14 PM