If the weight of the vehicle is on the springs, there will be a considerable amount of down pressure on the torque tube. You'll need to jack up the frame high enough to take the weight off the wheels and support the frame (not the rear axle) with jack stands before unbolting anything at the rear of the transmission. You might be able to do some of the preliminary work from inside the cab, but I'd strongly recommend recruiting some help before you get to the point of actually unbolting the torque tube and the U joint. A friend back in high school in the early 1960's got himself pinned under the torque tube when it slammed down during a transmission swap on a 47 Chevy car because he had the jack stands under the axle housing, not the frame. He got lucky, with only a few cuts and bruises.
Jerry
