Shaking as excellerating

I have a 02 chevy Tahoe 5.3 and when I put it from park to drive it makes a clank noise but however when I start driveing it,it shifts fine but when I get to 22 miles a hr. my steering wheel and whole vehicle shakes bad as I excellerate but if i let off the gas pedal it stops shaking but it only does it in 2 wheel drive. I put it in 4 wheel drive and its fine. I was told by 2 people they thought it was my U-Joints but i checked the short sdrive shaft and they seemed fine.another person mentioned the differentials. Any input would be greatly appreciated.