99 gmc sierra front end problems

i have a 99 gmc sierra 1500 4.8L and all of a sudden i am getting a bad vibration form the front end when excelerating, as soon as i let off the gas the vibrating quits. and thats the beggining. when i engage the 4 wheel drive the right front tire is locking up. can someone please help me?:banghead: any help is appreciated.