Here's the deal:
I've been driving around in the snow lately with all the melted chemicals and salt on the ground. Yesterday I was rounding a corner in 2wd; I came through some ice and noticed a scratching sound. It sounded as if I had an ice block lodged under my right front quarter-panel /frame/tire or rotor. I looked- there was nothing there.

The scratching sound only happens when I'm in 2wd with constant pressure on the gas between 1300-2100rpm. If I let off the gas - the sound goes away. If I put it in 4wd the sound goes away. If the truck is in 4wd and I switch it into 2wd on the fly - the sound does not come back. But, while driving in 4wd, after switching to 2wd (where the sound has stopped), if I come to a complete stop (staying in 2wd) and start again - the sound returns.

It's nuts. I don't have the time to rip it all apart and figure it out. Any suggestions?