my_chevy_4x4

whats up? i own a 2000 blazer, each time I hit 40 MPH my truck jerks/jumps like when your driving a MANUAL S-10 and each time you shift gears it does that jerk thing. now whenever i come to a stop sign or a red light, as i slow down almost to a dead stop my truck feels like down shifts or like a jump/jerk itself forward and theres a quick noise coming from the driveshaft area? but from me preventing it doing that, I shift in Nuetral before im at a dead stop and it wont jerk. When i put it back in Drive, it like jumps a couple times as i accelerate then it'll stop. And this only happens once each time i start my truck. My step dad told me it could be my U-Joints? (bearings inside?) need help on this one please. Thanks