I recently bought a '91 4Runner and like a fool I didn't try out the 4WD beforehand. Well, the other day I tried it out and as soon as I started moving there was a light clunking/grinding sound coming from the vicinity of my transmission. It isn't violent and the vehicle doesn't drive rough, it just sounds almost like a gear is flopping around spinning free. I haven't tested to see if my front wheels are actually engaging, but I was running smoothly on the street, no hesitation in the front wheels.

Where should I start looking to see what's wrong short of just tearing things open and looking for stripped gears?

 

I would check the Front diff. spin the front drivline by hand and feel for roughness. Try it in 4wd and in 2wd

Make sure its not the t-case and its not a hard fix
Or just a really bad u-joint.
Do some feel in around down there let us know what you find.