4 wheel drive clicking and knocking noise?

The 4wd issue sounds like a worn outer cv joint. Unlike a front wheel drive car, yours do not have torque applied to them when you're in 2wd. The torque is needed to cause them to bind and make noise. Your mechanic might run the truck in 4wd on a hoist, and listen with a stethoscope, but they still might not make noise since they're able to rotate freely. Another trick is to attach microphones, and listen to them while driving the truck in 4wd. The tool is called a "Chassis Ear".

The outer cv joints give very little trouble, so you might also look at the universal joints in the front drive shaft. The shaft is made up of two parts splined together. If the grease fitting has been ignored, the shaft could be binding when it changes length due to suspension travel. That won't happen in 2wd since it's not under pressure then.

As for the backfiring, I have a feeling it's not related to a sensor problem because restarting the engine isn't likely to change anything. There have been some problems with ignition switches, but the overheated contacts usually affect the radio / heater fan / power window circuit. That is a different circuit than the one that runs the engine. Still, repositioning the switch is the only obvious thing that's changing. The next time this happens, try moving the ignition switch just a little to see if the problem clears up. If it does, replace the switch.

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Based on the additional symptoms, and especially if this is a constant problem, suspect a plugged catalytic converter. No chemicals are going to solve this type of problem.

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There are two ways to tell if a cv joint is the cause of a clicking noise. One is to hang from the bottom of the truck while a friend drives the truck and drags you through the snow. A better method is to use a "Chassis Ear". That is a set of six microphones, a receiver, and a set of head phones. The microphones are clipped to strategic locations, then you switch between them and listen while driving. Most new car dealers have this tool but a lot of mechanics have never seen or heard of it. Some auto parts stores borrow or rent tools and they might have this one, otherwise it is available through the tool truck guys. I can post the link for it on the Mac Tools web site.

You can try to listen next to the joint while a helper runs the engine in gear. You'll need all four tires jacked up off the ground. Run the truck with the wheels turned fully to one side. Since the noise will typically only occur when there is a load on the joint, apply some pressure to the brake pedal. Use a block of wood under one front tire to make the other one spin so you can listen by it. That still might not be conclusive. Normally we just go by the sound.

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