1999 Chevy Tahoe Clanking Noise

only when putting truck into gear there is a clanck and only when driving there is a constant clanking noise coming from underneath middle of truck I think it maybe U-joints but not sure, and this is in rearwheel drive. When I select 4-Wheel the clanking is still there but it won't go back to rear wheel unless I shut off truck and restart then push button for rear wheel. Light has come on once on dash transfer case sevice