It sounds like you may have a spline issue. When you are in two wheel drive the front wheels drive the differential and it still spins and then spins the front driveshaft. If the splines between a CV shaft and the differential strip and the splines will start to grind and spin against each other when you are driving, this is easier for it than spinning the weight of the driveshaft and differential carrier. Then when you put it in four wheel drive and the transfer case drives the front driveshaft it will turn it at nearly the exact same speed as the front wheels are turning, so the spline noise stops. The CV shaft being driven by the wheels and the stub shaft being turned by the transfer case are turning at the same speed so they can't make noise. I hope this makes sense...

If the splines are worn between the CV shaft and stub shaft it won't be able to transmit power in four wheel drive so the front wheels won't drive. The stub shaft will just spin inside the CV shaft.

Slide under the truck and take a look at the area where the front CV shafts go to the differential housing. This is where the stripping normally happens. Check for looseness between the shaft and the differential but also look for rust dust coming out from the splines. If you see rust dust coming out that is from the splines wearing. You should find that one side of the truck will have this, probably the left since that is where the noise is coming from. When you identify which side is the problem you'll need to replace that CV shaft and the stub shaft that engages it. The stub shaft is the short shaft that goes into the differential and engages the side gears, and on the other end engages the CV shaft.

If you don't see any rust dust or can't see of feel any slop it could be that the CV shaft is stripped inside one of the boots. This would be less likely than stripped splines but definitely possible. I would take it apart and inspect the splines and go from there.