So, I drive a '96 T100 with almost 300,000 miles on it, it's held up well despite the age and abuse (I mean, it's a Toyota >.>), most problems I can diagnose with a little help from my dad, but he doesn't know 4x4s at all so this one has us stumped.

It started just after turning off the highway, I get a kinda of metallic popping sound when driving in 2 wheel drive- it sounds like it's coming from the front driver's side wheel. It's possible I'm imagining it but I also think I can feel it through the floor board. It seems to happen more frequently at higher speeds, but isn't super regular, I have not checked to see if it occurs while the truck is off the ground and I spin the wheels by hand. The sound goes away when I go in to 4 wheel drive, and returns if I drop back out to 2.

Anyone have any experience with this, or any advice on what I can check before I have to pay a mechanic to tell me what's wrong?

 

I know nothing about the T100, but I would imagine that you have the same vacuum ADD 4WD system that came on later first gen 4Runners and 2nd gen 4Runners. At 300k miles, I would check wheel bearings, also jack up each front wheel and spin them by hand, see if you can pinpoint where the noise is coming from. Try turning the steering wheel to get some stress on the CV axles when spinning the tires. Without having manual locking hubs (which I am assuming the T100 does not, but I could be wrong), there isn't much outboard of the CV axle shaft other than wheel bearings. So to me it's likely either the wheel bearings, CV axle, or inside the front differential. It could also be front driveshaft, but since you said it makes the noise only in 2WD, I'd rule this out although wouldn't hurt to great your u-joint and the cardan joint in the driveshaft.

Does your 4WD actually work? Meaning are you sure that when you put it in 4WD, that the front tires are spinning? The reason I ask is because most times when I've seen posts about front end noises, it seems that it usually happens in 4WD but not 2WD, and yours is the opposite.