A GMC 1/2 ton long bed has a short stub shaft between the transmission and the torque tube. It has a front and rear U joint and a bearing and bushing. It is very common for that area to give the noise you have.
Most of the time pinion bearings when going bad give their first noise on the pull, as that load is what it most receives.
After a while running with a pitted bearing will chew up the ball separator and the bearing will start to fall apart, then anything can happen as far as noise.
