Ive got a grinding/clanking noise when driving in 2wd. Noise not there when in 4wd. Has gotten much worse in the past few days and now it is humming while driving. No matter what the speed the grinding noise is still there. Even if I throw it in neutral and just coast its still there. Once again all these noises happen when im in 2wd not 4wd. Ive checked the brakes and rotors and things of that nature but im no mechanic. Im at a loss and would much appreciate some advise. Have any ideas? Thanks.