What would cause a loud grinding noise from the front left wheel in a '99 Pathfinder?

It started suddenly. I drove it to work and no noise, when I left 3 hours later, it sounded like the wheel was going to fall off! I drove it 300 yards and pulled over to look and nothing was apparent.

I drove it at 60 mph for 5 miles and it was VERY loud, but didn't seem to pull left or cause the engine to strain to get up to speed. I pulled into a rest stop and felt the hub/wheel and it wasn't hot.

I proceeded to drive it another 4 miles at 60mph, then at 35 - 40 mph for 2 miles.

I stopped at a red light and put it in 4wd to see if it changed AND NO NOISE!!! I drove about 1/2 mile with no noise, then took it out of 4wd and it came right back. I tried to shift it back to 4wd as I was moving and it didn't quiet down, it stayed loud.

I pulled over and stopped, then shifted into 4wd and it was quiet again.

Everything looks normal underneath and the tranfer case drive shaft and trans axles turn by hand when it is not in 4wd and idleing in park. HELP!