I have a '96 T100 and I've noticed it sounds like the front drivers side cv axle grinds when I turn. When I take it out of 4WH the grinding stops. Why is this? It's only in 4WH and only when I turn.

 

You say it only happens in high, but did you try low range? If it only does it in high range, that would be really weird and would suggest something in the transfer case. I suspect you just mean it only happens when in 4wd (high/low doesn't matter). First step I'd do is visually look at the cv axles on both sides, are the boots gone, popped out of joint? If it looks good, you might have to tear into it to find the problem. I'm personally guessing it's the manual hub/locker on the end of the cv axle that locks it to the hub. If that's the case, might be a good time to "upgrade" to manual lockers, if not you might find the plate style locker around for fairly cheap from someone that did the upgrade. You'll probably need a cv axle in either case since the splines at the end of the shaft needs to be in good shape or it will cause problems again.

With my guess/theory, when the truck is in 4x4, the axle has a lot more stresses on it, big time while turning. The front of the vehicle travels farther than the rear while turning, which means somethings has to give, normally it's the tires the slip. This is the case only in 4x4 because while in 4x4, the front and rear drive shafts are locked together, while in 2wd they can spin at different speeds since the front one is more or less disconnected.