Like I said, it wouldn't surprise me if it was caused by how much traction your front tires get on pavement, but to explore other options, lets check your t-case. since you have neither driveshaft connected right now, hop in your truck and fire it up, put it in 4wd, and first gear (make sure your drive shafts are disconnected or you'll hear more banging than you care to know about) and see if it still grinds. Who knows it could be the 4wd part of the t-case, kind of a long shot though.
